Question

I am a RoR programmer new to Python. I am trying to find the syntax that will allow me to set a variable to a specific value only if it wasn’t previously assigned. Basically I want:

# only if var1 has not been previously assigned

var1 = 4

Answer 1

This is a very different style of programming, but I always try to rewrite things that looked like

bar = None
if foo():
    bar = "Baz"

if bar is None:
    bar = "Quux"

into just:

if foo():
    bar = "Baz"
else:
    bar = "Quux"

That is to say, I try hard to avoid a situation where some code paths define variables but others don’t. In my code, there is never a path which causes an ambiguity of the set of defined variables (In fact, I usually take it a step further and make sure that the types are the same regardless of code path). It may just be a matter of personal taste, but I find this pattern, though a little less obvious when I’m writing it, much easier to understand when I’m later reading it.