pep8 minor annoyance

Question

In /tmp/spam.py:

n = 69

if n == True:
    print 'potato'

pep8 utility complains about this conditional:

wim@SDFA100461C:/tmp$ pep8 spam.py 
spam.py:3:6: E712 comparison to True should be 'if cond is True:' or 'if cond:'

• the first suggestion is wrong/"worse" according to pep8 itself
• the second suggestion changes the behaviour of the code

What is the best practice for a case where you actually do want to check equality with True? Is identity checking with True using is OK? Why does pep8 utility offer an alternative which is explicitly discouraged in pep8 itself?

Answer 1

If you really do need to check equality with True then use == and ignore PEP8, but in almost any conceivable case that isn't what you want.

If you want to know whether the value you have is one of the values Python considers to be true, use if cond:. If you want to know whether the value you have is the singleton value True then use is True, the booleans True and False are singletons so it is correct to use is in this situation.

If you need to test whether your object is the singleton True, but a linter or a code reviewer complains about is True, then isinstance(x, bool) and x is a behaviorally equivalent (but slower) substitute.

Checking x == True is a halfway house. It is true when x is True is true, and false for your case of x=69, but there are other objects which are not themselves True but for which x==True gives an unexpectedly true result such as 1 == True being true. (thanks @Ant).

So putting that together:

value of n:    True  1     69     False   0
-----------------------------------------------
expression     result
-----------------------------------------------
if n:          True  True  True   False   False
if n is True:  True  False False  False   False
if n==True:    True  True  False  False   False

Pick the row from that table which gives the results you really wanted (and the last one isn't it).