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pythonbooleanjythoncpythonpypy
ohmu
ohmu

Reputation: 19780

Boolean identity == True vs is True

It is standard convention to use if foo is None rather than if foo == None to test if a value is specifically None.

If you want to determine whether a value is exactly True (not just a true-like value), is there any reason to use if foo == True rather than if foo is True? Does this vary between implementations such as CPython (2.x and 3.x), Jython, PyPy, etc.?

Example: say True is used as a singleton value that you want to differentiate from the value 'bar', or any other true-like value:

if foo is True: # vs foo == True
    ...
elif foo == 'bar':
    ...

Is there a case where using if foo is True would yield different results from if foo == True?

NOTE: I am aware of Python booleans - if x:, vs if x == True, vs if x is True. However, it only addresses whether if foo, if foo == True, or if foo is True should generally be used to determine whether foo has a true-like value.

UPDATE: According to PEP 285 § Specification:

The values False and True will be singletons, like None.

Upvotes: 148

Views: 131093

Answers (8)

pegasus
pegasus

Reputation: 69

  • if x: This will invoke the x.__bool__() method to determine the truthiness of x.
  • if x == True: This uses the x.__eq__(True) method to compare x to the boolean value True.
  • if x is True: This checks if x is the boolean value True specifically, meaning x must be the exact same object as True (i.e., it shares the same memory address).

These three conditions can yield different results, as they check different aspects of x.

Upvotes: 0

tbrugere
tbrugere

Reputation: 1623

Using foo is True instead of foo == True (or just foo) if is most of the time not what you want.

I have seen foo is True used for checking that the parameter foo really was a boolean.

  1. It contradicts python's duck-typing philosophy (you should in general not check for types. A function acting differently with True than with other truthy values is counter-intuitive for a programmer who assumes duck-typing)
  2. Even if you want to check for types, it is better to do it explicity like :
def myFunction(foo):
    if not isinstance(foo, bool):
        raise ValueError("foo should be a boolean")
>>> myFunction(1)
Exception: ValueError "foo should be a boolean"

For several reasons:

  • Bool is the only type where the is operator will be equivalent to isinstance(a, bool) and a. The reason for that is the fact that True and False are singletons. In other words, this works because of a poorly known feature of python (especially when some tutorials teach you that True and False are just aliases for 1 and 0). The is operator is meant to check whether two objects are the same instance in memory. using it to check that something is a boolean is at best confusing.
  • If you use isinstance and the programmer was not aware that your function did not accept truthy-values, or if they are using numpy and forgot to cast their numpy-boolean to a python-boolean, they will know what they did wrong, and will be able to debug.

Compare with

def myFunction(foo):
    if foo is True:
       doSomething()
    else:
       doSomethingElse()

In this case, myFunction(1) not only does not raise an exception, but probably does the opposite of what it was expected to do. This makes for a hard to find bug in case someone was using a numpy boolean for example.

When should you use is True then ?

EDIT: Please don’t. It’s explicitely advised against by PEP 8 as mentioned in comments on another answer.

EDIT: this is bad practice, starting from 3.9, python raises a warning when you try to use is to compare with a literal. See @ JayDadhania's comment below. In conclusion is should not be used to compare to literals, only to check the equality of memory address.

Just don't use it. If you need to check for type, use isinstance.

Old paragraph:

Basically, use it only as a shorthand for isinstance(foo, bool) and foo

The only case I see is when you explicitely want to check if a value is true, and you will also check if the value is another truthy value later on. Examples include:

if foo is True:
   doSomething()
elif foo is False:
   doSomethingElse()
elif foo is 1: #EDIT: raises a warning, use == 1 instead
   doYetSomethingElse()
else:
   doSomethingElseEntirely()

Upvotes: 6

acushner
acushner

Reputation: 9946

edit: regarding:

Is there a case where using if foo is True would yield different results from if foo == True?

there is a case, and it's this:

In [24]: 1 is True
Out[24]: False

In [25]: 1 == True
Out[25]: True

additionally, if you're looking to use a singleton as a sentinel value, you can just create an object:

sentinel_time = object()

def f(snth):
    if snth is sentinel_time:
        print 'got em!'
f(sentinel_time)

you don't want to use if var == True:, you really want if var:.

imagine you have a list. you don't care if a list is "True" or not, you just want to know whether or not it's empty. so...

l = ['snth']
if l:
    print l

check out this post for what evaluates to False: Evaluation of boolean expressions in Python

Upvotes: 12

bruno desthuilliers
bruno desthuilliers

Reputation: 77912

is there any reason to use if foo == True rather than if foo is True?"

>>> d = True
>>> d is True
True
>>> d = 1
>>> d is True
False
>>> d == True
True
>>> d = 2
>>> d == True
False

Note that bool is a subclass of int, and that True has the integer value 1. To answer your question, if you want to check that some variable "is exactly True", you have to use the identity operator is. But that's really not pythonic... May I ask what's your real use case - IOW : why do you want to make a difference between True, 1 or any 'truth' value ?

Upvotes: 29

kadee
kadee

Reputation: 8884

Never use is True in combination with numpy (and derivatives such as pandas):

In[1]: import numpy as np
In[2]: a = np.array([1, 2]).any()
In[4]: a is True
Out[4]: False
In[5]: a == True
Out[5]: True

This was unexpected to me as:

In[3]: a
Out[3]: True

I guess the explanation is given by:

In[6]: type(a)
Out[6]: numpy.bool_

Upvotes: 52

Ferdinand Beyer
Ferdinand Beyer

Reputation: 67197

If you want to determine whether a value is exactly True (not just a true-like value), is there any reason to use if foo == True rather than if foo is True?

If you want to make sure that foo really is a boolean and of value True, use the is operator.

Otherwise, if the type of foo implements its own __eq__() that returns a true-ish value when comparing to True, you might end up with an unexpected result.

As a rule of thumb, you should always use is with the built-in constants True, False and None.

Does this vary between implementations such as CPython (2.x and 3.x), Jython, PyPy, etc.?

In theory, is will be faster than == since the latter must honor types' custom __eq__ implementations, while is can directly compare object identities (e.g., memory addresses).

I don't know the source code of the various Python implementations by heart, but I assume that most of them can optimize that by using some internal flags for the existence of magic methods, so I suspect that you won't notice the speed difference in practice.

Upvotes: 129

Mark Ransom
Mark Ransom

Reputation: 308490

Here's a test that allows you to see the difference between the 3 forms of testing for True:

for test in ([], [1], 0, 1, 2):
    print repr(test), 'T' if test else 'F', 'T' if test == True else 'F', 'T' if test is True else 'F'

[] F F F
[1] T F F
0 F F F
1 T T F
2 T F F

As you can see there are cases where all of them deliver different results.

Upvotes: 2

Kevin
Kevin

Reputation: 30161

Most of the time, you should not care about a detail like this. Either you already know that foo is a boolean (and you can thus use if foo), or you know that foo is something else (in which case there's no need to test). If you don't know the types of your variables, you may want to refactor your code.

But if you really need to be sure it is exactly True and nothing else, use is. Using == will give you 1 == True.

Upvotes: 0

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