if statement without a condition

Question

def f1(x,y):
      if x:    
          x = [1,2,3]
          x.append(4)
      else:
          x = 2
      return x + y

L1 = [1,2,3]
L2 = [55,66]
L3 = []
y = 3
prin         


        

Answer 1

Your if statement is equivalent to if bool(x): ... where Python first tries to look for a __nonzero__ method on x (__bool__ in Python 3) and if it cannot find it returns True per default unless x is None, False, has a __len__ method that returns zero, is an empty mapping or a numeric type with value zero.

Some examples:

>>> class A(object):
...     pass
... 
>>> bool(A())
True
>>> class B(object):
...     def __nonzero__(self): return False
... 
>>> bool(B())
False
>>> class C(object):
...     def __len__(self): return 0
... 
>>> bool(C())
False
>>> class D(object):
...     def __len__(self): return 0
...     def __nonzero__(self): return True
... 
>>> bool(D())
True