Python中的'True'和'False'

Question

I tried running this piece of code:

path = '/bla/bla/bla'

if path is True:
    print "True"
else:
    print "False"

And it prints False . I thought Python treats anything with value as True . Why is this happening?

Answer 1

is compares identity. A string will never be identical to a not-string.

== is equality. But a string will never be equal to either True or False .

You want neither.

path = '/bla/bla/bla'

if path:
    print "True"
else:
    print "False"

Answer 2

From http://docs.python.org/reference/expressions.html#boolean-operations :

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.

The key phrasing here that I think you are misunderstanding is "interpreted as false" or "interpreted as true". This does not mean that any of those values are identical to True or False, or even equal to True or False.

The expression '/bla/bla/bla' will be treated as true where a Boolean expression is expected (like in an if statement), but the expressions '/bla/bla/bla' is True and '/bla/bla/bla' == True will evaluate to False for the reasons in Ignacio's answer.

Answer 3

While the other posters addressed why is True does what it does, I wanted to respond to this part of your post:

I thought Python treats anything with value as True. Why is this happening?

Coming from Java, I got tripped up by this, too. Python does not treat anything with a value as True . Witness:

if 0:
    print("Won't get here")

This will print nothing because 0 is treated as False . In fact, zero of any numeric type evaluates to False . They also made decimal work the way you'd expect:

from decimal import *
from fractions import *

if 0 or 0.0 or 0j or Decimal(0) or Fraction(0, 1):
    print("Won't get here")

Here are the other value which evaluate to False :

if None or False or '' or () or [] or {} or set() or range(0):
    print("Won't get here")

Sources:

1. https://medium.com/@dawran6/python-truth-value-testing-is-awesome-dae6c23cc1c2
2. https://docs.python.org/3.7/library/stdtypes.html#truth-value-testing