python at mrabarnett
Jun 12, 2012, 11:25 AM
Post #4 of 12
(428 views)
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Re: using identifiers before they are defined
[In reply to]
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On 12/06/2012 18:53, Julio Sergio wrote:
> I'm puzzled with the following example, which is intended to be a part of a
> module, say "tst.py":
>
> a = something(5)
>
> def something(i):
> return i
>
>
>
> When I try:
>
> ->>> import tst
>
> The interpreter cries out:
>
> Traceback (most recent call last):
> File "<stdin>", line 1, in<module>
> File "tst.py", line 11, in<module>
> a = something(5)
> NameError: name 'something' is not defined
>
> I know that changing the order of the definitions will work, however there are
> situations in which referring to an identifier before it is defined is
> necessary, e.g., in crossed recursion.
>
> So I modified my module:
>
> global something
>
> a = something(5)
>
>
> def something(i):
> return i
>
>
> And this was the answer I got from the interpreter:
>
> ->>> import tst
>
> Traceback (most recent call last):
> File "<stdin>", line 1, in<module>
> File "tst.py", line 12, in<module>
> a = something(5)
> NameError: global name 'something' is not defined
>
>
> Do you have any comments?
>
In Python, "def" is a statement, not a declaration. It binds the body of
the function
to the name when the "def" statement is run.
A Python script is, basically, run from top to bottom, and both "def"
and "class" are actually statements, not declarations.
A function can refer to another function, even one that hasn't been
defined yet, provided that it has been defined by the time it is called.
For example, this:
def first():
second()
def second():
pass
first()
is OK because it defines function "first", then function "second", then
calls "first". By the time "first" calls "second", "second" has been
defined.
--
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