Question

In Javascript, I can assign a value like this so:

var i = p || 4

Obviously if p isn’t defined it will resort to 4. Is there a more elegant version of this operation in Python than a try: / except: combo?

try:
    i = p
except:
    i = 4

Answer 1

People sometime use Python’s or-operator for this purpose:

def f(p=None):
    i = p or 4
    ...

The relies on a “unique to Python” aspect of the or-operator to return the value that makes the expression true (rather than just returning True or False).

Another option that affords more flexibility is to use a conditional expression:

def f(p=None):
    i = p if p is not None else 4
    ...

If you just want to check to see if a variable is already defined, a try/except is the cleanest approach:

try:
    i = p
except NameError:
    pass

That being said, this would be atypical for Python and may be a hint that there is something wrong with the program design.

It is better to simply initialize a variable to a place-holder value such as None. In this regard, Python’s style is somewhat different from Javascript.