Question

I’ve come across some code that reads:

if bool(x):
    doSomething

I think that the following would do the same job:

if x:
    doSomething

The reference says that it evaluates the suite if the test expression

is found to be true

The reference says of Boolean expressions:

In the context of Boolean operations, and also when expressions are used by control flow statements are used
by control flow statements, the following values are interpreted as
false: False, None, numeric zero of all types, and empty strings and
containers … All other values are interpreted as true.

The reference says of the bool() function:

Convert a value to a Boolean, using the standard truth testing procedure

So are the two above identical or is there some extra subtlety to it?

Answer 1

if will use __nonzero__() if available, as does bool() when testing a value for truth. So yes, the behaviour is equivalent.

From the documentation:

In the context of Boolean operations, and also when expressions are
used by control flow statements, the following values are interpreted
as false: False, None, numeric zero of all types, and empty strings
and containers (including strings, tuples, lists, dictionaries, sets
and frozensets). All other values are interpreted as true. (See the
__nonzero__() special method for a way to change this.)

object.__nonzero__(self)

Called to implement truth value testing and the built-in operation bool(); should return False or True, or their integer equivalents 0 or
1. When this method is not defined, __len__() is called, if it is defined, and the object is considered true if its result is nonzero.
If a class defines neither __len__() nor __nonzero__(), all its
instances are considered true.