python array creation syntax [for in range]

Question

I came across the following syntax to create a python array. It is strange to me.

Can anyone explain it to me? And how should I learn this kind of syntax?

[str(index) for index in range(100)]

Answer 1

First of all, this is not an array. This is a list. Python does have built-in arrays, but they are rarely used (google the array module, if you're interested). The structure you see is called list comprehension. This is the fastest way to do vectorized stuff in pure Python. Let's get through some examples.

Simple list comprehensions are written this way: [item for item in iterable] - this will build a list containing all items of an iterable.

Actually, you can do something with each item using an expression or a function: [item**2 for item in iterable] - this will square each element, or [f(item) for item in iterable] - f is a function.

You can even add if and else statements like this [number for number in xrange(10) if not number % 2] - this will create a list of even numbers; ['even' if not number % 2 else 'odd' for number in range(10)] - this is how you use else statements.

You can nest list comprehensions [[character for character in word] for word in words] - this will create a list of lists. List comprehensions are similar to generator expressions, so you should google Python docs for additional information.

List comprehensions and generator expressions are among the most powerful and valuable Python features. Just start an interactive session and play for a while.

P.S.

There are other types of comprehensions that create sets and dictionaries. They use the same concept. Google them for additional information.