cost of len() and pep8 suggestion on sequence empty check

Question

If the complexity of python len() is O(1), why does pep8 suggest to use if seq: instead of if len(seq) == 0:

https://wiki.python.org/moin/TimeComplexity
https://www.python.org/dev/peps/pep-0008/#programming-recommendations

Isn't len(seq) == 0 more readable?

Answer 1

The former can handle both empty string and None. For example consider these two variables.

>>> s1 = ''
>>> s2 = None

Using the first method

def test(s):
    if s:
        return True
    else:
        return False

>>> test(s1)
False
>>> test(s2)
False

Now using len

def test(s):
    if len(s) == 0:
        return True
    else:
        return False

>>> test(s1)
True
>>> test(s2)
Traceback (most recent call last):
  File "<pyshell#13>", line 1, in <module>
    test(s2)
  File "<pyshell#11>", line 2, in test
    if len(s) == 0:
TypeError: object of type 'NoneType' has no len()

So in terms of performance, both will be O(1), but the truthiness test (the first method) is more robust in that it handles None in addition to empty strings.

Answer 2

The fact the len is O(1) is only a convention. There may be sequences/collections where len is not O(1) at that point checking len(x) == 0 could take asymptotically more time.

The fact is: the question "is the size of this collection equal to k?" and "is the collection empty?" are fundamentally different problems. The first is answered by len(x) == k and the latter by bool(x) [i.e. is x truthy]. Hence if you want to check if the collection is empty you just use if collection. They just happen to coincide if k == 0.

Generally you should use the most specific term to describe exactly what you are doing. Writing len(x) == 0 may just be a typo for len(x) == 10 with a missing 1 while getting if x wrong is harder.