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Alexander Hess 2019-11-20 11:00:24 +01:00
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03_conditionals.ipynb
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"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",
"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**."
]
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"`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 re-use 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",
"`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."
]
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"So the following expression regards *four* objects in memory: *One* `list` object holding ten pointers to *three* other objects."
"So the following expression regards *four* objects in memory: *One* `list` object holding ten references to *three* other objects."
]
},
{