On Thu, Nov 27, 2014 at 9:33 AM, Steven D'Aprano <st...@pearwood.info> wrote:
> On Thu, Nov 27, 2014 at 09:00:48AM -0600, boB Stepp wrote:
[...]
> But there is a subtlety that you may not expect:
>
> py> class Tricky:
> ... print(x)
> ... x = "inner"
> ... print(x)
> ...
> outer
> inner
Actually, this is what you had me primed to expect. However, ...
> So although classes introduce a new scope, they are not like functions.
> In a function, the above would lead to an error (try it and see).
>>> def tricky_func():
print(x)
x = "inner"
print(x)
>>> tricky_func()
Traceback (most recent call last):
File "<pyshell#20>", line 1, in <module>
tricky_func()
File "<pyshell#19>", line 2, in tricky_func
print(x)
UnboundLocalError: local variable 'x' referenced before assignment
This surprised me! So I did:
>>> def tricky_func2():
y = x
print(x)
>>> tricky_func2()
outer
So why does not print(x) see the global x and instead looks for the
local x? And why is this different between classes and functions?
> * Every module is a separate scope.
>
> * `def` and `class` introduce new scopes.
>
> * Other indented blocks (for, while, if etc.) do not.
Alan's reference to indentation level had me trying to prove the
opposite--unsuccessfully.
> * But lambda expressions do.
>
> * Generator expressions have their own scope too.
>
> * In Python 3 only, so do list comprehensions (and dict and set
> comprehensions). But not in Python 2.
This is good to know this difference as at work I'm in Python 2.4.4
and at home Python 3.4
--
boB
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