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{
"cells": [
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"**Note**: Click on \"*Kernel*\" > \"*Restart Kernel and Clear All Outputs*\" in [JupyterLab](https://jupyterlab.readthedocs.io/en/stable/) *before* reading this notebook to reset its output. If you cannot run this file on your machine, you may want to open it [in the cloud <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_mb.png\">](https://mybinder.org/v2/gh/webartifex/intro-to-python/develop?urlpath=lab/tree/03_conditionals/00_content.ipynb)."
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"# Chapter 3: Conditionals & Exceptions"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"We analyzed every aspect of the `average_evens()` function in [Chapter 2 <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_nb.png\">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/develop/02_functions/00_content.ipynb) except for the `if`-related parts. While it does what we expect it to, there is a whole lot more to learn by taking it apart. In particular, the `if` may occur within both a **statement** or an **expression**, analogous as to how a noun in a natural language can be the subject of *or* an object in a sentence. What is common to both usages is that it leads to code being executed for *parts* of the input only. It is a way of controlling the **flow of execution** in a program.\n",
"\n",
"After deconstructing `if` in the first part of this chapter, we take a close look at a similar concept, namely handling **exceptions**."
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"## Boolean Expressions"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Any expression that is either true or not is called a **boolean expression**. It is such simple true-or-false observations about the world on which mathematicians, and originally philosophers, base their rules of reasoning: They are studied formally in the field of [propositional logic <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_wiki.png\">](https://en.wikipedia.org/wiki/Propositional_calculus).\n",
"\n",
"A trivial example involves the equality operator `==` that evaluates to either `True` or `False` depending on its operands \"comparing equal\" or not."
]
},
{
"cell_type": "code",
"execution_count": 1,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 1,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 == 42"
]
},
{
"cell_type": "code",
"execution_count": 2,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 2,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 == 123"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"The `==` operator handles objects of *different* types: Because of that, it implements a notion of equality in line with how humans think of things being equal or not. After all, `42` and `42.0` are different $0$s and $1$s for a computer and other programming languages may say `False` here! Technically, this is yet another example of operator overloading."
]
},
{
"cell_type": "code",
"execution_count": 3,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 3,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 == 42.0"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"There are, however, cases where the `==` operator seems to not work intuitively. [Chapter 5 <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_nb.png\">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/develop/05_numbers/00_content.ipynb#Imprecision) provides more insights into this \"bug.\""
]
},
{
"cell_type": "code",
"execution_count": 4,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 4,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 == 42.000000000000001"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"## The `bool` Type"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"`True` and `False` are built-in *objects* of type `bool`."
]
},
{
"cell_type": "code",
"execution_count": 5,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 5,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"True"
]
},
{
"cell_type": "code",
"execution_count": 6,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"94478067031520"
]
},
"execution_count": 6,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"id(True)"
]
},
{
"cell_type": "code",
"execution_count": 7,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"bool"
]
},
"execution_count": 7,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"type(True)"
]
},
{
"cell_type": "code",
"execution_count": 8,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 8,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"False"
]
},
{
"cell_type": "code",
"execution_count": 9,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"94478067031552"
]
},
"execution_count": 9,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"id(False)"
]
},
{
"cell_type": "code",
"execution_count": 10,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"bool"
]
},
"execution_count": 10,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"type(False)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Let's not confuse the boolean `False` with `None`, another built-in object! We saw the latter before in [Chapter 2 <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_nb.png\">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/develop/02_functions/00_content.ipynb#Function-Definitions) as the *implicit* return value of a function without a `return` statement.\n",
"\n",
"We might think of `None` indicating a \"maybe\" or even an \"unknown\" answer; however, for Python, there are no \"maybe\" or \"unknown\" objects, as we see further below!\n",
"\n",
"Whereas `False` is of type `bool`, `None` is of type `NoneType`. So, they are unrelated! On the contrary, as both `True` and `False` are of the same type, we could call them \"siblings.\""
]
},
{
"cell_type": "code",
"execution_count": 11,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [],
"source": [
"None"
]
},
{
"cell_type": "code",
"execution_count": 12,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"94478066920032"
]
},
"execution_count": 12,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"id(None)"
]
},
{
"cell_type": "code",
"execution_count": 13,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"NoneType"
]
},
"execution_count": 13,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"type(None)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"### Singletons"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"`True`, `False`, and `None` have the property that they each exist in memory only *once*. Objects designed this way are so-called **singletons**. This **[design pattern <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_wiki.png\">](https://en.wikipedia.org/wiki/Design_Patterns)** was originally developed to keep a program's memory usage at a minimum. It may only be employed in situations where we know that an object does *not* mutate its value (i.e., to reuse the bag analogy from [Chapter 1 <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_nb.png\">](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/develop/01_elements/00_content.ipynb#Objects-vs.-Types-vs.-Values), no flipping of $0$s and $1$s in the bag is allowed). In languages \"closer\" to the memory like C, we would have to code this singleton logic ourselves, but Python has this built in for *some* types.\n",
"\n",
"We verify this with either the `is` operator or by comparing memory addresses."
]
},
{
"cell_type": "code",
"execution_count": 14,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 14,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"a = True\n",
"b = True\n",
"\n",
"a is b"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"To contrast this, we create *two* `789` objects."
]
},
{
"cell_type": "code",
"execution_count": 15,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 15,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"a = 789\n",
"b = 789\n",
"\n",
"a is b"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"So the following expression regards *four* objects in memory: *One* `list` object holding six references to *three* other objects."
]
},
{
"cell_type": "code",
"execution_count": 16,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"[True, False, None, True, False, None]"
]
},
"execution_count": 16,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"[True, False, None, True, False, None]"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"## Relational Operators"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"The equality operator is only one of several **relational** (i.e., \"comparison\") **operators** who all evaluate to a `bool` object."
]
},
{
"cell_type": "code",
"execution_count": 17,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 17,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 == 123"
]
},
{
"cell_type": "code",
"execution_count": 18,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 18,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 != 123 # \"not equal to\""
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"The \"less than\" `<` or \"greater than\" `>` operators mean \"*strictly* less than\" or \"*strictly* greater than\" but may be combined with the equality operator into just `<=` and `>=`. This is a shortcut for using the logical `or` operator as described in the next section."
]
},
{
"cell_type": "code",
"execution_count": 19,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 19,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 < 123"
]
},
{
"cell_type": "code",
"execution_count": 20,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 20,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 <= 123"
]
},
{
"cell_type": "code",
"execution_count": 21,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 21,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 > 123"
]
},
{
"cell_type": "code",
"execution_count": 22,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 22,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 >= 123"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"## Logical Operators"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Boolean expressions may be combined or negated with the **logical operators** `and`, `or`, and `not` to form new boolean expressions. This may be done repeatedly to obtain boolean expressions of arbitrary complexity.\n",
"\n",
"Their usage is similar to how the equivalent words are used in everyday English:\n",
"\n",
"- `and` evaluates to `True` if *both* operands evaluate to `True` and `False` otherwise,\n",
"- `or` evaluates to `True` if either one *or* both operands evaluate to `True` and `False` otherwise, and\n",
"- `not` evaluates to `True` if its *only* operand evaluates to `False` and vice versa."
]
},
{
"cell_type": "code",
"execution_count": 23,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [],
"source": [
"a = 42\n",
"b = 87"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Relational operators have **[higher precedence <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/reference/expressions.html#operator-precedence)** over logical operators. So the following expression means what we intuitively think it does."
]
},
{
"cell_type": "code",
"execution_count": 24,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 24,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"a > 5 and b <= 100"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"However, sometimes, it is good to use *parentheses* around each operand for clarity."
]
},
{
"cell_type": "code",
"execution_count": 25,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 25,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"(a > 5) and (b <= 100)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"This is especially so when several logical operators are combined."
]
},
{
"cell_type": "code",
"execution_count": 26,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 26,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"a <= 5 or not b > 100"
]
},
{
"cell_type": "code",
"execution_count": 27,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 27,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"(a <= 5) or not (b > 100)"
]
},
{
"cell_type": "code",
"execution_count": 28,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 28,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"(a <= 5) or (not (b > 100)) # no need to \"over do\" it"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"For even better readability, some practitioners suggest to *never* use the `>` and `>=` operators (cf., [source](https://llewellynfalco.blogspot.com/2016/02/dont-use-greater-than-sign-in.html); note that the included example is written in [Java <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_wiki.png\">](https://en.wikipedia.org/wiki/Java_%28programming_language%29) where `&&` means `and` and `||` means `or`).\n",
"\n",
"We may **chain** operators if the expressions that contain them are combined with the `and` operator. For example, the following two cells implement the same logic, where the second is a lot easier to read."
]
},
{
"cell_type": "code",
"execution_count": 29,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 29,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"5 < a and a < 87"
]
},
{
"cell_type": "code",
"execution_count": 30,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 30,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"5 < a < 87"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"### Truthy vs. Falsy"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"The operands of a logical operator do not need to be *boolean* expressions but may be *any* expression. If an operand does *not* evaluate to an object of type `bool`, Python automatically casts it as such. Then, Pythonistas say that the expression is evaluated in a boolean context.\n",
"\n",
"For example, any non-zero numeric object is cast as `True`. While this behavior allows writing more concise and thus more \"beautiful\" code, it may also be a source of confusion.\n",
"\n",
"So, `(a - 40)` is cast as `True` and then the overall expression evaluates to `True` as well."
]
},
{
"cell_type": "code",
"execution_count": 31,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 31,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"(a - 40) and (b < 100)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Whenever we are unsure how Python evaluates a non-boolean expression in a boolean context, the [bool() <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/library/functions.html#bool) built-in allows us to do it ourselves. [bool() <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/library/functions.html#bool), like [int() <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/library/functions.html#int), is yet another *constructor*."
]
},
{
"cell_type": "code",
"execution_count": 32,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 32,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool(a - 40)"
]
},
{
"cell_type": "code",
"execution_count": 33,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 33,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool(a - 42)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Let's keep in mind that negative numbers also evaluate to `True`!"
]
},
{
"cell_type": "code",
"execution_count": 34,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 34,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool(a - 44)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"In a boolean context, `None` is cast as `False`! So, `None` is *not* a \"maybe\" answer but a \"no.\""
]
},
{
"cell_type": "code",
"execution_count": 35,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 35,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool(None)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Another good rule to know is that container types (e.g., `list`) evaluate to `False` whenever they are empty and `True` if they hold at least one element."
]
},
{
"cell_type": "code",
"execution_count": 36,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 36,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool([])"
]
},
{
"cell_type": "code",
"execution_count": 37,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 37,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool([False])"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"With `str` objects, the empty `\"\"` evaluates to `False`, and any other to `True`."
]
},
{
"cell_type": "code",
"execution_count": 38,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 38,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool(\"\")"
]
},
{
"cell_type": "code",
"execution_count": 39,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 39,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool(\"Lorem ipsum dolor sit amet.\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Pythonistas use the terms **truthy** or **falsy** to describe a non-boolean expression's behavior when evaluated in a boolean context."
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"### Short-Circuiting"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"When evaluating expressions involving the `and` and `or` operators, Python follows the **[short-circuiting <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_wiki.png\">](https://en.wikipedia.org/wiki/Short-circuit_evaluation)** strategy: Once it is clear what the overall truth value is, no more operands are evaluated, and the result is *immediately* returned.\n",
"\n",
"Also, if such expressions are evaluated in a non-boolean context, the result is returned as is and *not* cast as a `bool` type.\n",
"\n",
"The two rules can be summarized as:\n",
"\n",
"- `a or b`: If `a` is truthy, it is returned *without* evaluating `b`. Otherwise, `b` is evaluated *and* returned.\n",
"- `a and b`: If `a` is falsy, it is returned *without* evaluating `b`. Otherwise, `b` is evaluated *and* returned.\n",
"\n",
"The rules may also be chained or combined.\n",
"\n",
"Let's look at a couple of examples below. To visualize which operands are evaluated, we define a helper function `expr()` that prints out the only argument it is passed before returning it."
]
},
{
"cell_type": "code",
"execution_count": 40,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"def expr(arg):\n",
" \"\"\"Print and return the only argument.\"\"\"\n",
" print(\"Arg:\", arg)\n",
" return arg"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"With the `or` operator, the first truthy operand is returned."
]
},
{
"cell_type": "code",
"execution_count": 41,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"1"
]
},
"execution_count": 41,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"0 or 1"
]
},
{
"cell_type": "code",
"execution_count": 42,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 0\n",
"Arg: 1\n"
]
},
{
"data": {
"text/plain": [
"1"
]
},
"execution_count": 42,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(0) or expr(1) # both operands are evaluated"
]
},
{
"cell_type": "code",
"execution_count": 43,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"1"
]
},
"execution_count": 43,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"1 or 2"
]
},
{
"cell_type": "code",
"execution_count": 44,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 1\n"
]
},
{
"data": {
"text/plain": [
"1"
]
},
"execution_count": 44,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(1) or expr(2) # 2 is not evaluated"
]
},
{
"cell_type": "code",
"execution_count": 45,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"1"
]
},
"execution_count": 45,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"0 or 1 or 2"
]
},
{
"cell_type": "code",
"execution_count": 46,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 0\n",
"Arg: 1\n"
]
},
{
"data": {
"text/plain": [
"1"
]
},
"execution_count": 46,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(0) or expr(1) or expr(2) # 2 is not evaluated"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"If all operands are falsy, the last one is returned."
]
},
{
"cell_type": "code",
"execution_count": 47,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 47,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"False or [] or 0"
]
},
{
"cell_type": "code",
"execution_count": 48,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: False\n",
"Arg: []\n",
"Arg: 0\n"
]
},
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 48,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(False) or expr([]) or expr(0) # all operands are evaluated"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"With the `and` operator, the first falsy operand is returned."
]
},
{
"cell_type": "code",
"execution_count": 49,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 49,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"0 and 1"
]
},
{
"cell_type": "code",
"execution_count": 50,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 0\n"
]
},
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 50,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(0) and expr(1) # 1 is not evaluated"
]
},
{
"cell_type": "code",
"execution_count": 51,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 51,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"1 and 0"
]
},
{
"cell_type": "code",
"execution_count": 52,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 1\n",
"Arg: 0\n"
]
},
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 52,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(1) and expr(0) # both operands are evaluated"
]
},
{
"cell_type": "code",
"execution_count": 53,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 53,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"1 and 0 and 2"
]
},
{
"cell_type": "code",
"execution_count": 54,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 1\n",
"Arg: 0\n"
]
},
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 54,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(1) and expr(0) and expr(2) # 2 is not evaluated"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"If all operands are truthy, the last one is returned."
]
},
{
"cell_type": "code",
"execution_count": 55,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"3"
]
},
"execution_count": 55,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"1 and 2 and 3"
]
},
{
"cell_type": "code",
"execution_count": 56,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 1\n",
"Arg: 2\n",
"Arg: 3\n"
]
},
{
"data": {
"text/plain": [
"3"
]
},
"execution_count": 56,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(1) and expr(2) and expr(3)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"The crucial takeaway is that Python does *not* necessarily evaluate *all* operands and, therefore, our code should never rely on that assumption."
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"## The `if` Statement"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"To write useful programs, we need to control the flow of execution, for example, to react to user input. The logic by which a program follows the rules from the \"real world\" is referred to as **[business logic <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_wiki.png\">](https://en.wikipedia.org/wiki/Business_logic)**.\n",
"\n",
"One language feature to do so is the `if` statement (cf., [reference <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/reference/compound_stmts.html#the-if-statement)). It consists of:\n",
"\n",
"- *one* mandatory `if`-clause,\n",
"- an *arbitrary* number of `elif`-clauses (i.e., \"else if\"), and\n",
"- an *optional* `else`-clause.\n",
"\n",
"The `if`- and `elif`-clauses each specify one *boolean* expression, also called **condition** here, while the `else`-clause serves as the \"catch everything else\" case.\n",
"\n",
"In contrast to our intuitive interpretation in natural languages, only the code in *one* of the alternatives, also called **branches**, is executed. To be precise, it is always the code in the first clause whose condition evaluates to `True`.\n",
"\n",
"In terms of syntax, the header lines end with a colon, and the code blocks are indented. Formally, any statement that is written across several lines is called a **[compound statement <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/reference/compound_stmts.html#compound-statements)**, the code blocks are called **suites** and belong to one header line, and the term **clause** refers to a header line and its suite as a whole. So far, we have seen three compound statements: `for`, `if`, and `def`. On the contrary, **[simple statements <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/reference/simple_stmts.html#simple-statements)**, for example, `=`, `del`, or `return`, are written on *one* line.\n",
"\n",
"As an example, let's write code that checks if a randomly drawn number is divisible by `2`, `3`, both, or none. The code should print out a customized message for each of the *four* cases."
]
},
{
"cell_type": "code",
"execution_count": 57,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [],
"source": [
"numbers = [7, 11, 8, 5, 3, 12, 2, 6, 9, 10, 1, 4]"
]
},
{
"cell_type": "code",
"execution_count": 58,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [],
"source": [
"import random"
]
},
{
"cell_type": "code",
"execution_count": 59,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(789)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"#### \"Wrong Logic\" Example: Is the number divisible by `2`, `3`, both, or none?"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"It turns out that translating this task into code is not so trivial. Whereas the code below looks right, it is *incorrect*. The reason is that the order of the `if`-, `elif`-, and `else`-clauses matters."
]
},
{
"cell_type": "code",
"execution_count": 60,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"6 is divisible by 2 only\n"
]
}
],
"source": [
"number = random.choice(numbers)\n",
"\n",
"if number % 2 == 0:\n",
" print(number, \"is divisible by 2 only\")\n",
"elif number % 3 == 0:\n",
" print(number, \"is divisible by 3 only\")\n",
"elif number % 2 == 0 and number % 3 == 0:\n",
" print(number, \"is divisible by 2 and 3\")\n",
"else:\n",
" print(number, \"is divisible by neither 2 nor 3\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"#### \"Correct Logic\" Example: Is the number divisible by `2`, `3`, both, or none?"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"As a number divisible by both `2` and `3` is always a special (i.e., narrower) case of a number being divisible by either `2` or `3` on their own, we must check for that condition first. The order of the two latter cases is not important as they are mutually exclusive. Below is a correct implementation of the program."
]
},
{
"cell_type": "code",
"execution_count": 61,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(789)"
]
},
{
"cell_type": "code",
"execution_count": 62,
"metadata": {
"code_folding": [],
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"6 is divisible by 2 and 3\n"
]
}
],
"source": [
"number = random.choice(numbers)\n",
"\n",
"if number % 3 == 0 and number % 2 == 0:\n",
" print(number, \"is divisible by 2 and 3\")\n",
"elif number % 3 == 0:\n",
" print(number, \"is divisible by 3 only\")\n",
"elif number % 2 == 0:\n",
" print(number, \"is divisible by 2 only\")\n",
"else:\n",
" print(number, \"is divisible by neither 2 nor 3\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"#### \"Concise Logic\" Example: Is the number divisible by `2`, `3`, both, or none?"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"A minor improvement could be to replace `number % 3 == 0 and number % 2 == 0` with the conciser `number % 6 == 0`. However, this has no effect on the order that is still essential for the code's correctness."
]
},
{
"cell_type": "code",
"execution_count": 63,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(789)"
]
},
{
"cell_type": "code",
"execution_count": 64,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"6 is divisible by 2 and 3\n"
]
}
],
"source": [
"number = random.choice(numbers)\n",
"\n",
"if number % 6 == 0:\n",
" print(number, \"is divisible by 2 and 3\")\n",
"elif number % 3 == 0:\n",
" print(number, \"is divisible by 3 only\")\n",
"elif number % 2 == 0:\n",
" print(number, \"is divisible by 2 only\")\n",
"else:\n",
" print(number, \"is divisible by neither 2 nor 3\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"#### Only the `if`-clause is mandatory"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Often, we only need a reduced form of the `if` statement.\n",
"\n",
"For example, below we **inject** code to print a message at random."
]
},
{
"cell_type": "code",
"execution_count": 65,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"You read this as often as you see heads when tossing a coin\n"
]
}
],
"source": [
"if random.random() > 0.5:\n",
" print(\"You read this as often as you see heads when tossing a coin\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"#### Common Use Case: A binary Choice"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"More often than not, we model a **binary choice**. Then, we only need to write an `if`- and an `else`-clause."
]
},
{
"cell_type": "code",
"execution_count": 66,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(789)"
]
},
{
"cell_type": "code",
"execution_count": 67,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"6 is even\n"
]
}
],
"source": [
"number = random.choice(numbers)\n",
"\n",
"if number % 2 == 0:\n",
" print(number, \"is even\")\n",
"else:\n",
" print(number, \"is odd\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"To write the condition even conciser, we may take advantage of Python's implicit casting and leave out the `== 0`. However, then we *must* exchange the two suits! The `if`-clause below means \"If the `number` is odd\" in plain English. That is the opposite of the `if`-clause above."
]
},
{
"cell_type": "code",
"execution_count": 68,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(789)"
]
},
{
"cell_type": "code",
"execution_count": 69,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"6 is even\n"
]
}
],
"source": [
"number = random.choice(numbers)\n",
"\n",
"if number % 2: # Note the opposite meaning!\n",
" print(number, \"is odd\")\n",
"else:\n",
" print(number, \"is even\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"#### \"Hard to read\" Example: Nesting `if` Statements"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"We may **nest** `if` statements to control the flow of execution in a more granular way. Every additional layer, however, makes the code *less* readable, in particular, if we have more than one line per code block.\n",
"\n",
"For example, the code cell below implements an [A/B Testing <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_wiki.png\">](https://en.wikipedia.org/wiki/A/B_testing) strategy where half the time a \"complex\" message is shown to a \"user\" while in the remaining times an \"easy\" message is shown. To do so, the code first \"tosses a coin\" and then checks a randomly drawn `number`."
]
},
{
"cell_type": "code",
"execution_count": 70,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(789)"
]
},
{
"cell_type": "code",
"execution_count": 71,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"6 can be divided by 2 without a rest\n"
]
}
],
"source": [
"number = random.choice(numbers)\n",
"\n",
"# Coin is heads.\n",
"if random.random() > 0.5:\n",
" if number % 2 == 0:\n",
" print(number, \"can be divided by 2 without a rest\")\n",
" else:\n",
" print(number, \"divided by 2 results in a non-zero rest\")\n",
"# Coin is tails.\n",
"else:\n",
" if number % 2 == 0:\n",
" print(number, \"is even\")\n",
" else:\n",
" print(number, \"is odd\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"#### \"Easy to read\" Example: Flattening nested `if` Statements"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"A way to make this code more readable is to introduce **temporary variables** *in combination* with the `and` operator to **flatten** the branching logic. The `if` statement then reads almost like plain English. In contrast to many other languages, creating variables is a computationally *cheap* operation in Python (i.e., only a reference is created) and also helps to document the code *inline* with meaningful variable names.\n",
"\n",
"Flattening the logic *without* temporary variables could lead to *more* sub-expressions in the conditions be evaluated than necessary. Do you see why?"
]
},
{
"cell_type": "code",
"execution_count": 72,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(789)"
]
},
{
"cell_type": "code",
"execution_count": 73,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"6 can be divided by 2 without a rest\n"
]
}
],
"source": [
"number = random.choice(numbers)\n",
"\n",
"coin_is_heads = random.random() > 0.5\n",
"number_is_even = number % 2 == 0\n",
"\n",
"if coin_is_heads and number_is_even:\n",
" print(number, \"can be divided by 2 without a rest\")\n",
"elif coin_is_heads and not number_is_even:\n",
" print(number, \"divided by 2 results in a non-zero rest\")\n",
"elif not coin_is_heads and number_is_even:\n",
" print(number, \"is even\")\n",
"else:\n",
" print(number, \"is odd\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"## The `if` Expression"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"When an `if` statement assigns an object to a variable according to a true-or-false condition (i.e., a binary choice), there is a shortcut: We assign the variable the result of a so-called **[conditional expression <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/reference/expressions.html#conditional-expressions)**, or `if` expression for short, instead.\n",
"\n",
"Think of a situation where we evaluate a piece-wise functional relationship $y = f(x)$ at a given $x$, for example:"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"$\n",
"y = f(x) =\n",
"\\begin{cases}\n",
"0, \\text{ if } x \\le 0 \\\\\n",
"x, \\text{ otherwise}\n",
"\\end{cases}\n",
"$"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Of course, we could use an `if` statement as above to do the job. Yet, this is rather lengthy."
]
},
{
"cell_type": "code",
"execution_count": 74,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"3"
]
},
"execution_count": 74,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"x = 3\n",
"\n",
"if x <= 0:\n",
" y = 0\n",
"else:\n",
" y = x\n",
"\n",
"y"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"On the contrary, the `if` expression fits into one line. The main downside is a potential loss in readability, in particular, if the functional relationship is not that simple. Also, some practitioners do *not* like that the condition is in the middle of the expression."
]
},
{
"cell_type": "code",
"execution_count": 75,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"3"
]
},
"execution_count": 75,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"x = 3\n",
"\n",
"y = 0 if x <= 0 else x\n",
"\n",
"y"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"In this example, however, the most elegant solution is to use the built-in [max() <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/library/functions.html#max) function."
]
},
{
"cell_type": "code",
"execution_count": 76,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"3"
]
},
"execution_count": 76,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"x = 3\n",
"\n",
"y = max(0, x)\n",
"\n",
"y"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"## The `try` Statement"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"In the previous two chapters, we encountered a couple of *runtime* errors. A natural urge we might have after reading about conditional statements is to write code that somehow reacts to the occurrence of such exceptions.\n",
"\n",
"Consider a situation where we are given some user input that may contain values that cause problems. To illustrate this, we draw a random integer between `0` and `5`, and then divide by this number. Naturally, we see a `ZeroDivisionError` in 16.6% of the cases."
]
},
{
"cell_type": "code",
"execution_count": 77,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(123)"
]
},
{
"cell_type": "code",
"execution_count": 78,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"ename": "ZeroDivisionError",
"evalue": "division by zero",
"output_type": "error",
"traceback": [
"\u001b[0;31m---------------------------------------------------------------------------\u001b[0m",
"\u001b[0;31mZeroDivisionError\u001b[0m Traceback (most recent call last)",
"\u001b[0;32m<ipython-input-78-7996bd5496ba>\u001b[0m in \u001b[0;36m<module>\u001b[0;34m\u001b[0m\n\u001b[1;32m 1\u001b[0m \u001b[0muser_input\u001b[0m \u001b[0;34m=\u001b[0m \u001b[0mrandom\u001b[0m\u001b[0;34m.\u001b[0m\u001b[0mchoice\u001b[0m\u001b[0;34m(\u001b[0m\u001b[0;34m[\u001b[0m\u001b[0;36m0\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m1\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m2\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m3\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m4\u001b[0m\u001b[0;34m,\u001b[0m \u001b[0;36m5\u001b[0m\u001b[0;34m]\u001b[0m\u001b[0;34m)\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[1;32m 2\u001b[0m \u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0;32m----> 3\u001b[0;31m \u001b[0;36m1\u001b[0m \u001b[0;34m/\u001b[0m \u001b[0muser_input\u001b[0m\u001b[0;34m\u001b[0m\u001b[0;34m\u001b[0m\u001b[0m\n\u001b[0m",
"\u001b[0;31mZeroDivisionError\u001b[0m: division by zero"
]
}
],
"source": [
"user_input = random.choice([0, 1, 2, 3, 4, 5])\n",
"\n",
"1 / user_input"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"With the compound `try` statement (cf., [reference <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/reference/compound_stmts.html#the-try-statement)), we can **handle** any *runtime* error.\n",
"\n",
"In its simplest form, it comes with just two clauses: `try` and `except`. The following tells Python to execute the code in the `try`-clause, and if *anything* goes wrong, continue in the `except`-clause instead of **raising** an error to us. Of course, if nothing goes wrong, the `except`-clause is *not* executed."
]
},
{
"cell_type": "code",
"execution_count": 79,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(123)"
]
},
{
"cell_type": "code",
"execution_count": 80,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Something went wrong\n"
]
}
],
"source": [
"user_input = random.choice([0, 1, 2, 3, 4, 5])\n",
"\n",
"try:\n",
" print(\"The result is\", 1 / user_input)\n",
"except:\n",
" print(\"Something went wrong\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"However, it is good practice *not* to handle *any* possible exception but only the ones we may *expect* from the code in the `try`-clause. The reason for that is that we do not want to risk *suppressing* an exception that we do *not* expect. Also, the code base becomes easier to understand as we communicate what could go wrong during execution in an *explicit* way to the (human) reader. Python comes with a lot of [built-in exceptions <img height=\"12\" style=\"display: inline-block\" src=\"../static/link/to_py.png\">](https://docs.python.org/3/library/exceptions.html#concrete-exceptions) that we should familiarize ourselves with.\n",
"\n",
"Another good practice is to always keep the code in the `try`-clause short to not *accidentally* handle an exception we do *not* want to handle.\n",
"\n",
"In the example, we are dividing numbers and may expect a `ZeroDivisionError`."
]
},
{
"cell_type": "code",
"execution_count": 81,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(123)"
]
},
{
"cell_type": "code",
"execution_count": 82,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Something went wrong\n"
]
}
],
"source": [
"user_input = random.choice([0, 1, 2, 3, 4, 5])\n",
"\n",
"try:\n",
" print(\"The result is\", 1 / user_input)\n",
"except ZeroDivisionError:\n",
" print(\"Something went wrong\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Often, we may have to run some code *independent* of an exception occurring, for example, to close a connection to a database. To achieve that, we add a `finally`-clause to the `try` statement.\n",
"\n",
"Similarly, we may have to run some code *only if* no exception occurs, but we do not want to put it in the `try`-clause as per the good practice mentioned above. To achieve that, we add an `else`-clause to the `try` statement.\n",
"\n",
"To showcase everything together, we look at one last example. It is randomized: So, run the cell several times and see for yourself."
]
},
{
"cell_type": "code",
"execution_count": 83,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [],
"source": [
"random.seed(123)"
]
},
{
"cell_type": "code",
"execution_count": 84,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Oops. Division by 0. How does that work?\n",
"I am always printed\n"
]
}
],
"source": [
"user_input = random.choice([0, 1, 2, 3, 4, 5])\n",
"\n",
"try:\n",
" result = 1 / user_input\n",
"except ZeroDivisionError:\n",
" print(\"Oops. Division by 0. How does that work?\")\n",
"else:\n",
" print(\"The result is\", result)\n",
"finally:\n",
" print(\"I am always printed\")"
]
}
],
"metadata": {
"kernelspec": {
"display_name": "Python 3",
"language": "python",
"name": "python3"
},
"language_info": {
"codemirror_mode": {
"name": "ipython",
"version": 3
},
"file_extension": ".py",
"mimetype": "text/x-python",
"name": "python",
"nbconvert_exporter": "python",
"pygments_lexer": "ipython3",
"version": "3.8.6"
},
"livereveal": {
"auto_select": "code",
"auto_select_fragment": true,
"scroll": true,
"theme": "serif"
},
"toc": {
"base_numbering": 1,
"nav_menu": {},
"number_sections": false,
"sideBar": true,
"skip_h1_title": true,
"title_cell": "Table of Contents",
"title_sidebar": "Contents",
"toc_cell": false,
"toc_position": {
"height": "calc(100% - 180px)",
"left": "10px",
"top": "150px",
"width": "384px"
},
"toc_section_display": false,
"toc_window_display": false
}
},
"nbformat": 4,
"nbformat_minor": 4
}
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