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{
"cells": [
{
"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](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/02_functions.ipynb) except for the `if`-related parts. While it seems to do what we expect it to, there is a whole lot more we learn from 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 is *either* 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 our first 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 \"statements\" about the world on which mathematicians, and originally philosophers, base their rules of reasoning: They are studied formally in the field of [propositional logic](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": "-"
}
},
"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: 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 might 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 even well-behaved Python does not make us happy. [Chapter 5](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/05_numbers.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": "skip"
}
},
"source": [
"`True` and `False` are built-in *objects* of type `bool`."
]
},
{
"cell_type": "code",
"execution_count": 5,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"93918328484832"
]
},
"execution_count": 5,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"id(True)"
]
},
{
"cell_type": "code",
"execution_count": 6,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"93918328484800"
]
},
"execution_count": 6,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"id(False)"
]
},
{
"cell_type": "code",
"execution_count": 7,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"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": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"bool"
]
},
"execution_count": 8,
"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](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/02_functions.ipynb#Function-Definition) as the *implicit* return value of a function without a `return` statement.\n",
"\n",
"We might think of `None` in a boolean context 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": 9,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [],
"source": [
"None"
]
},
{
"cell_type": "code",
"execution_count": 10,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"93918328471792"
]
},
"execution_count": 10,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"id(None)"
]
},
{
"cell_type": "code",
"execution_count": 11,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"NoneType"
]
},
"execution_count": 11,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"type(None)"
]
},
{
"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](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](https://nbviewer.jupyter.org/github/webartifex/intro-to-python/blob/master/01_elements.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": 12,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 12,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"True is True"
]
},
{
"cell_type": "code",
"execution_count": 13,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 13,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"id(True) == id(True)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"So the following expression regards *four* objects in memory: *One* `list` object holding ten references to *three* other objects."
]
},
{
"cell_type": "code",
"execution_count": 14,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"[True, False, None, None, None, True, False, None, None, None]"
]
},
"execution_count": 14,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"[True, False, None, None, None, True, False, None, None, 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 boolean object."
]
},
{
"cell_type": "code",
"execution_count": 15,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 15,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 == 123"
]
},
{
"cell_type": "code",
"execution_count": 16,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 16,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 != 123 # = \"not equal to\"; other languages may use \"<>\""
]
},
{
"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": 17,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 17,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 < 123"
]
},
{
"cell_type": "code",
"execution_count": 18,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 18,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 <= 123 # same as 42 < 123 or 42 == 123; cf., next section"
]
},
{
"cell_type": "code",
"execution_count": 19,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 19,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 > 123"
]
},
{
"cell_type": "code",
"execution_count": 20,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 20,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"42 >= 123 # same as 42 > 123 or 42 == 123; cf., next section"
]
},
{
"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. Of course, this may be done *recursively* as well 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* sub-expressions evaluate to `True` and `False` otherwise,\n",
"- `or` evaluates to `True` if either one *or* both sub-expressions evaluate to `True` and `False` otherwise, and\n",
"- `not` evaluates to `True` if its *only* sub-expression evaluates to `False` and vice versa."
]
},
{
"cell_type": "code",
"execution_count": 21,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [],
"source": [
"x = 42\n",
"y = 87"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Relational operators have **[higher precedence](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": 22,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 22,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"x > 5 and y <= 100"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"However, sometimes, it is good to use *parentheses* around each sub-expression for clarity."
]
},
{
"cell_type": "code",
"execution_count": 23,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 23,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"(x > 5) and (y <= 100)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"This is especially the case when several logical operators are combined."
]
},
{
"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": [
"x <= 5 or not y > 100"
]
},
{
"cell_type": "code",
"execution_count": 25,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 25,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"(x <= 5) or not (y > 100)"
]
},
{
"cell_type": "code",
"execution_count": 26,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 26,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"(x <= 5) or (not (y > 100)) # but 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](https://en.wikipedia.org/wiki/Java_%28programming_language%29) where `&&` means `and` and `||` means `or`).\n",
"\n",
"Python allows **chaining** relational operators that 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": 27,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 27,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"(5 < x) and (x < 21)"
]
},
{
"cell_type": "code",
"execution_count": 28,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 28,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"5 < x < 21"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"source": [
"### Truthy vs. Falsy"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"The operands of the logical operators do not need to be *boolean* expressions but may be *any* expression. If a sub-expression does *not* evaluate to an object of type `bool`, Python automatically casts it as such.\n",
"\n",
"For example, any non-zero numeric object becomes `True`. While this behavior allows writing more concise and thus more \"beautiful\" code, it is also a common source of confusion.\n",
"\n",
"So, `(x - 9)` is cast as `True` and then the overall expression evaluates to `True` as well."
]
},
{
"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": [
"(x - 9) and (y < 100) # = 33 and (y < 100) = 33 and True"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Whenever we are unsure as to how Python evaluates a non-boolean expression in a boolean context, the [bool()](https://docs.python.org/3/library/functions.html#bool) built-in allows us to check it ourselves."
]
},
{
"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": [
"bool(x - 9) # = bool(33)"
]
},
{
"cell_type": "code",
"execution_count": 31,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 31,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool(x - 42) # = bool(0)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"Keep in mind that negative numbers also evaluate to `True`."
]
},
{
"cell_type": "code",
"execution_count": 32,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 32,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool(x - 99) # = bool(-57)"
]
},
{
"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": 33,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 33,
"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 `True` whenever they are *not* empty and `False` if they are."
]
},
{
"cell_type": "code",
"execution_count": 34,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"data": {
"text/plain": [
"False"
]
},
"execution_count": 34,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool([])"
]
},
{
"cell_type": "code",
"execution_count": 35,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"True"
]
},
"execution_count": 35,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"bool([False])"
]
},
{
"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](https://en.wikipedia.org/wiki/Short-circuit_evaluation)** strategy: Once it is clear what the overall truth value is, no more sub-expressions 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",
"- `x or y`: If `x` is truthy, it is returned *without* evaluating `y`. Otherwise, `y` is evaluated *and* returned.\n",
"- `x and y`: If `x` is falsy, it is returned *without* evaluating `y`. Otherwise, `y` 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 sub-expressions 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": 36,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"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 sub-expression is returned."
]
},
{
"cell_type": "code",
"execution_count": 37,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 0\n",
"Arg: 1\n"
]
},
{
"data": {
"text/plain": [
"1"
]
},
"execution_count": 37,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(0) or expr(1)"
]
},
{
"cell_type": "code",
"execution_count": 38,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 1\n"
]
},
{
"data": {
"text/plain": [
"1"
]
},
"execution_count": 38,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(1) or expr(2) # 2 is not evaluated due to short-circuiting"
]
},
{
"cell_type": "code",
"execution_count": 39,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 0\n",
"Arg: 1\n"
]
},
{
"data": {
"text/plain": [
"1"
]
},
"execution_count": 39,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(0) or expr(1) or expr(2) # 2 is not evaluated due to short-circuiting"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"If all sub-expressions are falsy, the last one is returned."
]
},
{
"cell_type": "code",
"execution_count": 40,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: False\n",
"Arg: []\n",
"Arg: 0\n"
]
},
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 40,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(False) or expr([]) or expr(0)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"With the `and` operator, the first falsy sub-expression is returned."
]
},
{
"cell_type": "code",
"execution_count": 41,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 0\n"
]
},
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 41,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(0) and expr(1) # 1 is not evaluated due to short-circuiting"
]
},
{
"cell_type": "code",
"execution_count": 42,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 1\n",
"Arg: 0\n"
]
},
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 42,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(1) and expr(0)"
]
},
{
"cell_type": "code",
"execution_count": 43,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 1\n",
"Arg: 0\n"
]
},
{
"data": {
"text/plain": [
"0"
]
},
"execution_count": 43,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(1) and expr(0) and expr(2) # 2 is not evaluated due to short-circuiting"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"If all sub-expressions are truthy, the last one is returned."
]
},
{
"cell_type": "code",
"execution_count": 44,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Arg: 1\n",
"Arg: 2\n"
]
},
{
"data": {
"text/plain": [
"2"
]
},
"execution_count": 44,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"expr(1) and expr(2)"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"The crucial takeaway is that Python does *not* necessarily evaluate *all* sub-expressions 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](https://en.wikipedia.org/wiki/Business_logic)**, even if \"real world\" refers to the domain of mathematics and not a business application.\n",
"\n",
"One language feature to do so is the `if` statement (cf., [reference](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** in this context, 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 a **[compound statement](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](https://docs.python.org/3/reference/simple_stmts.html#simple-statements)**, for example, `=`, `del`, or `return`, are written on *one* line."
]
},
{
"cell_type": "code",
"execution_count": 45,
"metadata": {
"code_folding": [],
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [],
"source": [
"z = 101"
]
},
{
"cell_type": "code",
"execution_count": 46,
"metadata": {
"code_folding": [],
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"z is positive but odd\n"
]
}
],
"source": [
"if (z % 2 == 0) and (z > 0):\n",
" print(\"z is even and positive\")\n",
"elif z % 2 == 0:\n",
" print(\"z is even but negative\")\n",
"elif z > 0:\n",
" print(\"z is positive but odd\")\n",
"else:\n",
" print(\"z is neither even nor positive\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"In many situations, we only need a reduced form of the `if` statement.\n",
"\n",
"We could **inject** code only at random, for example, to implement an [A/B testing](https://en.wikipedia.org/wiki/A/B_testing) strategy."
]
},
{
"cell_type": "code",
"execution_count": 47,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [],
"source": [
"import random"
]
},
{
"cell_type": "code",
"execution_count": 48,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [],
"source": [
"if random.random() > 0.5:\n",
" print(\"You read this just as often as you see heads when tossing a coin\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"More often than not, we model a **binary choice**."
]
},
{
"cell_type": "code",
"execution_count": 49,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"z is positive\n"
]
}
],
"source": [
"if z > 0:\n",
" print(\"z is positive\")\n",
"else:\n",
" print(\"z is negative\")"
]
},
{
"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",
"The code cell below *either* checks if a number is even or odd *or* if it is positive or negative."
]
},
{
"cell_type": "code",
"execution_count": 50,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"z is positive\n"
]
}
],
"source": [
"if random.random() > 0.5:\n",
" if z % 2: # no need to write out the \"== 0\"\n",
" print(\"z is odd\")\n",
" else:\n",
" print(\"z is even\")\n",
"else:\n",
" if z > 0:\n",
" print(\"z is positive\")\n",
" else:\n",
" print(\"z is negative\")"
]
},
{
"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 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": 51,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"z is positive\n"
]
}
],
"source": [
"check_oddness = (random.random() > 0.5)\n",
"is_odd = (z % 2)\n",
"is_positive = (z > 0)\n",
"\n",
"if check_oddness and is_odd:\n",
" print(\"z is odd\")\n",
"elif check_oddness and not is_odd:\n",
" print(\"z is even\")\n",
"elif not check_oddness and is_positive:\n",
" print(\"z is positive\")\n",
"else:\n",
" print(\"z is negative\")"
]
},
{
"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](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": [
"$\n",
"y = f(x) =\n",
"\\begin{cases}\n",
"0, \\text{ if } x \\le 0 \\\\\n",
"x^2, \\text{ otherwise}\n",
"\\end{cases}\n",
"$"
]
},
{
"cell_type": "code",
"execution_count": 52,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [],
"source": [
"x = 3"
]
},
{
"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": 53,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [],
"source": [
"if x <= 0:\n",
" y = 0\n",
"else:\n",
" y = x ** 2"
]
},
{
"cell_type": "code",
"execution_count": 54,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"data": {
"text/plain": [
"9"
]
},
"execution_count": 54,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"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": 55,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [],
"source": [
"y = 0 if x <= 0 else x ** 2"
]
},
{
"cell_type": "code",
"execution_count": 56,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"9"
]
},
"execution_count": 56,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"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()](https://docs.python.org/3/library/functions.html#max) function."
]
},
{
"cell_type": "code",
"execution_count": 57,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [],
"source": [
"y = max(0, x) ** 2"
]
},
{
"cell_type": "code",
"execution_count": 58,
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"outputs": [
{
"data": {
"text/plain": [
"9"
]
},
"execution_count": 58,
"metadata": {},
"output_type": "execute_result"
}
],
"source": [
"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. All we need is a way to formulate a condition for that.\n",
"\n",
"For sure, this is such a common thing to do that Python provides a language construct for it, namely the compound `try` statement (cf., [reference](https://docs.python.org/3/reference/compound_stmts.html#the-try-statement)).\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": 59,
"metadata": {
"slideshow": {
"slide_type": "slide"
}
},
"outputs": [],
"source": [
"user_input = 0"
]
},
{
"cell_type": "code",
"execution_count": 60,
"metadata": {
"slideshow": {
"slide_type": "-"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Something went wrong\n"
]
}
],
"source": [
"try:\n",
" 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 reasoning why this is done is a bit involved. We only remark that the codebase becomes easier to understand as we communicate to any human reader what could go wrong during execution in an *explicit* way. Python comes with a lot of [built-in exceptions](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": 61,
"metadata": {
"slideshow": {
"slide_type": "fragment"
}
},
"outputs": [
{
"name": "stdout",
"output_type": "stream",
"text": [
"Something went wrong\n"
]
}
],
"source": [
"try:\n",
" 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. To spice it up a bit, we randomize the input. So run the cell several times and see for yourself."
]
},
{
"cell_type": "code",
"execution_count": 62,
"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": [
"divisor = random.choice([0, 1])\n",
"\n",
"try:\n",
" 1 / divisor\n",
"except ZeroDivisionError:\n",
" print(\"Oops. Division by 0. How does that work?\")\n",
"else:\n",
" print(\"Yes, division worked smoothly.\")\n",
"finally:\n",
" print(\"I am always printed\")"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"## TL;DR"
]
},
{
"cell_type": "markdown",
"metadata": {
"slideshow": {
"slide_type": "skip"
}
},
"source": [
"- **boolean expressions** evaluate to either `True` or `False`\n",
"- **relational operators** compare operands according to \"human\" interpretations\n",
"- **logical operators** combine boolean sub-expressions to more \"complex\" expressions\n",
"- the **conditional statement** allows **controlling** the **flow of execution** depending on some **conditions**\n",
"- a **conditional expression** is a short form of a conditional statement\n",
"- **exception handling** is also a common way of **controlling** the **flow of execution**, in particular, if we have to be prepared for bad input data"
]
}
],
"metadata": {
"kernelspec": {
"display_name": "Python 3",
"language": "python",
"name": "python3"
},
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