Python scoping and if-then statements

Question

I want to set a variable based on the value of another variable in python. But when I ask for the value of the variable set in the if-then statement, it can't find the variable, obviously because it is now out of scope.

Consider the following:

>>> a = True
>>> if a:
...     b=1
... else:
...     b=2
>>> print b
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'b' is not defined

Is there a pythonic way to write this? For example, the following code works, but is it "the right way" to write it, or is there a better way?

>>> b = None
>>> if a:
...     b=1
... else:
...     b=2
... 
>>> b
1

What do you mean? Your first example works fine in all versions of Python. — , Apr 13 '15 at 18:45
I agree with iCodez -- 'b' is defined in python2.7 when I try your example. — bmhkim, Apr 13 '15 at 18:46
It really really really should work as you've written it. Scoping in Python is done on a function level, so b should be in scope. — DavidW, Apr 13 '15 at 18:46
Could you show actual interpreter output, copy-pasted from an actual interpreter session? Your examples have visible inconsistencies with the actual way the Python interpreter requires you to provide input. — user2357112 supports Monica, Apr 13 '15 at 18:48
It was because I put it in wrong, that's why it didn't work. sorry guys :( — djhaskin987, Apr 13 '15 at 19:44
@djhaskin987 Check my answer for a simpler way of assigning like this. — Anshul Goyal, Apr 14 '15 at 04:37

score 2 · Accepted Answer · edited May 23 '17 at 12:22

2

For a simple assignment, you could use python's equivalent of ternary operator :

>>> a = True
>>> b = 1 if a else 2
>>> b
1

Also, I can't reproduce your given example,

In [1]: a = True

In [2]: if a:
   ...:     b = 1
   ...: else:
   ...:     b = 2
   ...:     

In [3]: print b
1

edited May 23 '17 at 12:22

Community

1
1

answered Apr 13 '15 at 18:44

Anshul Goyal

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In my years of Python, I've never seen an concise assignment like this. Is this even in the documentation? Thanks for the eye opener. – Malik Brahimi Apr 13 '15 at 18:48
You can use that everywhere: assignments, comprehensions, function arguments... it's quite excellent. :) – TigerhawkT3 Apr 13 '15 at 18:49
2

@MalikBrahimi: [Here's the relevant documentation.](https://docs.python.org/2/reference/expressions.html#conditional-expressions) – user2357112 supports Monica Apr 13 '15 at 18:50

Python scoping and if-then statements

1 Answers1