Is there any difference between `if bool(x)` and `if x` in Python?

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

Objects are implicitly converted to bool type when they are placed in an if statement. So, for most purposes, there's no difference between x and bool(x) in an if statement. However, you will incur extra overhead if you call bool() because you are making a function call. Here's a quick test to demonstrate this:

In [7]: %timeit if(''): pass
10000000 loops, best of 3: 21.5 ns per loop

In [8]: %timeit if(bool('')): pass
1000000 loops, best of 3: 235 ns per loop

Answer 2

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.

Answer 3

any Object that you put in an if statement will be converted to a bool based on some internal python checker, normally not an issue, there is no difference between bool(x) and (x) when inside an if statement.

however, the reason bool(x) exists is for cases such as:

return bool(x)

which would return "true or false" based on the object.