If not, what is the best way to do this?
Right now I'm doing (for a django project):
if not 'thing_for_purpose' in request.session: request.session['thing_for_purpose'] = 5 but its pretty awkward. In Ruby it would be:
request.session['thing_for_purpose'] ||= 5 which is much nicer.
Jon-Eric's answer's is good for dicts, but the title seeks a general equivalent to ruby's ||= operator.
A common way to do something like ||= in Python is
x = x or new_value 
x has been previously defined (for example set to None in a class initialization method).
x = request.session['thing_for_purpose'] or 'my default' will raise a KeyError if 'thing_for_purpose' is not defined in the dict. The other answers in this thread with "get" on a dict are IMHO better answersPrecise 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. (What's 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).
dict has setdefault().
So if request.session is a dict:
request.session.setdefault('thing_for_purpose', 5) 
Setting a default makes sense if you're doing it in a middleware or something, but if you need a default value in the context of one request:
request.session.get('thing_for_purpose', 5) # gets a default bonus: here's how to really do an ||= in Python.
def test_function(self, d=None): 'a simple test function' d = d or {} # ... do things with d and return ... In general, you can use dict[key] = dict.get(key, 0) + val.
nil(false in boolean context) this idiom is often used exactly for the purpose sean has used it. The idiom only works though if, of course,nilandfalseare not legitimate values in the hash (which is very often true, so the idiom is fine)//=which you find in perl. I think the||=also originates from Perl.