Python “all” function with conditional generator expression returning True. Why?

问题

Can anyone help me understand why the following Python script returns True?

x = ''
y = all(i == ' ' for i in x)
print(y)

I imagine it's something to do with x being a zero-length entity, but cannot fully comprehend.

回答1:

all() always returns True unless there is an element in the sequence that is False.

Your loop produces 0 items, so True is returned.

This is documented:

Return True if all elements of the iterable are true (or if the iterable is empty).

Emphasis mine.

Similarly, any() will always return False, unless an element in the sequence is True, so for empty sequences, any() returns the default:

>>> any(True for _ in '')
False

回答2:

As the documentation states, what all does is:

Return True if all elements of the iterable are true (or if the iterable is empty).

来源：https://stackoverflow.com/questions/19257821/python-all-function-with-conditional-generator-expression-returning-true-why