Special condition syntax with parentheses and brackets [duplicate]

Question

Is there any explanation about the following Python condition syntax?

>>> a = 10
>>> s = (0, 1)[a<10]
>>> print s
0
>>> a = -10
>>> s = (0, 1)[a<10]
>>> print s
1

It seems to work like an if statement. Why does this work?

Answer 1

In Python, bool is a subclass of int.

>>> issubclass(bool, int)
True

In other word, False is equal to 0, and True is equal to 1:

>>> False == 0
True
>>> True == 1
True

So they can be used as a index:

>>> ['No', 'Yes'][False]  # ['No', 'Yes'][0]
'No'
>>> ['No', 'Yes'][True]   # ['No', 'Yes'][1]
'Yes'

The expression a < 10 yields True or False based on the value of a. So (0, 1)[a < 10] will yield 0 or 1 accordingly.

Answer 2

(0, 1) is a 2-element tuple. You can access its values using the index accessors [0] and [1] like this:

>>> (0, 1)[0]
0
>>> (0, 1)[1]
1

Now, in Python, the boolean values True and False are actually instances of int (because bool is a subtype of int):

>>> issubclass(bool, int)
True
>>> isinstance(True, int)
True
>>> isinstance(False, int)
True

The int values of True and False are 1 and 0:

>>> True == 1
True
>>> False == 0
True

So you can actually use them two access the elements in your 2-element tuple:

>>> (0, 1)[False]
0
>>> (0, 1)[True]
1

And that explains why specifying a condition there—which returns a boolean—will work as well.

This is also mentioned in the documentation (emphasis mine):

Boolean values are the two constant objects False and True. They are used to represent truth values (although other values can also be considered false or true). In numeric contexts (for example when used as the argument to an arithmetic operator), they behave like the integers 0 and 1, respectively.