[Python-Dev] LOAD_NAME & classes (original) (raw)

Guido van Rossum guido@python.org
Wed, 24 Apr 2002 00:49:07 -0400

[Tim]

It's hard to know what could really help more. If Greg Wilson is still running newcomer experiments, I'd like to see what newcomers have to say about this:

x = 2 def f(): print x # A x = 3 f() print x # B A: What do you think should happen when the print at A executes? B: " " " " " " " " " " B " ? What I suspect, but don't know, is that a majority of newcomers who expect A to print 2 also expect B to print 3. That is, that they're thinking x is a global variable, and have no conception of local variables in mind.

That's not a fair experiment until after you've given them a good concept of local and global variables without name conflicts. On the one hand, the importance of having local variables at all isn't clear if there are no name conflicts (until you introduce recursion, which I would definitely introduce much later, despite Matthias Felleisen's teachings :-). But on the other hand, it's easy to show that after

>>> def f():
...     greeting = "hello world"
...     print greeting
...
>>> f()
hello world
>>> print greeting
NameError: greeting
>>>

there's no global variable 'greeting', which can then be used to explain that variable assignments create local variables. You don't need to explain the reason for this feature at this point; that'll be clear by the time you've explained the rest. Next you can explain the global statement. This appears irrelevant to the proposed experiment, but it's useful to take away fears that you can't change globals at all: people coming from other languages will know that that is important, and worry how Python deals with this. After that you can show how name conflicts are handled in the "normal" case, where you shadow a global in a function by assigning to it and then using it, without a global statement. At some point you should also point out that if you don't change a global, you can use in a function it without the global statement. This comes so natural that it's easy to gloss over, especially when the "global" in question is a function or an imported module; but it is an important feature.

THEN you are ready for the experiment Tim proposes above.

--Guido van Rossum (home page: http://www.python.org/~guido/)