自我陈述什么时候是真的，什么时候是假的？

Question

Can someone explain this if self.cards condition? When will it be True and when will it be False?

def __init__(self):
    self.cards = []

def __str__(self):
    if self.cards:
        rep = ""
        for card in self.cards:
            rep += str(card) + " "
    else:
        rep = "<empty>"
    return rep

Answer 1

First, you should probably show us when self.cards is used for the first time.
Assuming it is some sort of a container( list , set or dict ) it will be true if there are elements in it and false if it is empty.

Answer 2

Any object can be tested for truth value in Python. The following values are considered false:

None

False

zero of any numeric type, for example, 0 , 0L , 0.0 , 0j .

any empty sequence, for example, '' , () , [] .

any empty mapping, for example, {} .

instances of user-defined classes, if the class defines a __nonzero__() or __len__() method, when that method returns the integer zero or bool value False .

All other values are considered true — so objects of many types are always true.

In this case cards is False when it is empty because it is a list. When the object is created, __init__() creates the cards empty list, so that if statement's condition is always False when the object is created.