Does Python have an or equals function like ||= in Ruby?

Does Python have an or equals function like ||= in Ruby?

Jon-Erics answers is good for dicts, but the title seeks a general equivalent to rubys ||= operator.

A common way to do something like ||= in Python is

x = x or new_value

dict has setdefault().

So if request.session is a dict:

request.session.setdefault(thing_for_purpose, 5)

Does Python have an or equals function like ||= in Ruby?

Precise answer: No. Python does not have a single built-in operator op that can translate x = x or y into x op y.

But, it almost does. The bitwise or-equals operator (|=) will function as described above if both operands are being treated as booleans, with a caveat. (Whats the caveat? Answer is below of course.)

First, the basic demonstration of functionality:

x = True
x    
Out[141]: True

x |= True
x    
Out[142]: True

x |= False
x    
Out[143]: True

x &= False
x    
Out[144]: False

x &= True
x    
Out[145]: False

x |= False
x    
Out[146]: False

x |= True
x   
Out[147]: True

The caveat is due python not being strictly-typed, and thus even if the values are being treated as booleans in an expression they will not be short-circuited if given to a bitwise operator. For example, suppose we had a boolean function which clears a list and returns True iff there were elements deleted:

def  my_clear_list(lst):
    if not lst:
        return False
    else:
        del lst[:]
        return True

Now we can see the short-circuited behavior as so:

x = True
lst = [1, 2, 3]
x = x or my_clear_list(lst)
print(x, lst)

Output: True [1, 2, 3]

However, switching the or to a bitwise or (|) removes the short-circuit, so the function my_clear_list executes.

x = True
lst = [1, 2, 3]
x = x | my_clear_list(lst)
print(x, lst)

Output: True []

Above, x = x | my_clear_list(lst) is equivalent to x |= my_clear_list(lst).