Python conditional assignment operator

I'm surprised no one offered this answer. It's not as "built-in" as Ruby's ||= but it's basically equivalent and still a one-liner:

foo = foo if 'foo' in locals() else 'default'

Of course, locals() is just a dictionary, so you can do:

foo = locals().get('foo', 'default')

I would use

x = 'default' if not x else x

Much shorter than all of your alternatives suggested here, and straight to the point. Read, "set x to 'default' if x is not set otherwise keep it as x." If you need None, 0, False, or "" to be valid values however, you will need to change this behavior, for instance:

valid_vals = ("", 0, False) # We want None to be the only un-set valuex = 'default' if not x and x not in valid_vals else x

This sort of thing is also just begging to be turned into a function you can use everywhere easily:

setval_if = lambda val: 'default' if not val and val not in valid_vals else val

at which point, you can use it as:

>>> x = None # To set it to something not valid>>> x = setval_if(x) # Using our special function is short and sweet now!>>> print x # Let's check to make sure our None valued variable actually got set'default'

Finally, if you are really missing your Ruby infix notation, you could overload ||=| (or something similar) by following this guy's hack: http://code.activestate.com/recipes/384122-infix-operators/

No, the replacement is:

try:   vexcept NameError:   v = 'bla bla'

However, wanting to use this construct is a sign of overly complicated code flow. Usually, you'd do the following:

try:   v = complicated()except ComplicatedError: # complicated failed   v = 'fallback value'

and never be unsure whether v is set or not. If it's one of many options that can either be set or not, use a dictionary and its get method which allows a default value.