Comprehension

Python has a special kind of expression called a comprehension. Comprehensions are variations of the special syntax for creating dictionaries, lists, and sets.

Let's look at some examples. Here we see a list comprehension:

capitals = [x.upper() for x in example_list] 

What this expression does is that it creates a new list containing the uppercase versions of the words in the old list.

The first part after the opening square bracket is an x.upper() expression. This expression describes how to derive a member of the new list from a member of the old list. After that is the for keyword, then the name of the x variable we used in the first expression to represent the values from the old list. Then, the keyword is followed by the example_list expression that gives us the old list and the closing square bracket. The code output is as follows:

The dictionary and set comprehensions are very similar. If we want to use both the key and the value of an existing dictionary in a comprehension, we need to use the dict.items function, and dictionary comprehensions need to specify both the key and value separated by a colon, as shown in this example:

squares = {k: v ** 2 for k, v in example_dict.items()} 

As shown in the following screenshot, notice that the resulting data type depends on what sort of comprehension we used, not on what sort of data structure we used as the source of data:

We can use a list comprehension to create a list of data pulled from the values of a dictionary, for example, or, as we did here, we can use the set comprehension to create a set.

Tuples are slightly different, but only slightly. A tuple comprehension would look exactly like a different syntactic element called a generator expression. The code example for tuple comprehension is as follows:

Python's designers hate ambiguity; so instead, if we want the equivalent of a tuple comprehension, we pass a generator expression to a tuple constructor.

That's it for this quick introduction to Python's built-in data structures. In the next section, we're going to look at some useful, but possibly surprising, traits of functions and classes that are significantly different from C, C++, or Java.