Easy questions from a python beginner

[Terry Reedy]

On 7/12/2010 4:48 AM, bart.c wrote:

>> >>> def foo():
>> print("Before:", locals())
>> x = 0
>> print("After:", locals())
>>
>>
>> >>> foo()
>> Before: {}
>> After: {'x': 0}
>
> That's interesting. So in Python, you can't tell what local variables a
> function has just by looking at it's code:

You are being fooled by the multiple meanings of the overloaded term 
'variable'. Thing are clearer with the two terms 'name' and 'namespace' 
(a set of name-object bindings).

Before compiling function code, an interpreter must read it and make a 
classified list of *names*. The interpreter has to know the fixed set of 
local names in order to know what to do with assignment statements. If 
the interpreter can tell how many local names there are, so can you.

Before a function begins execution, all parameter names must be bound, 
with no arguments left over. At that point, the local namespace consists 
of those parameter-object bindings. During the execution of the 
function, bindings may be added and deleted. The size of the local 
namespace varies.

> def foo(day):
> if day=="Tuesday":
> x=0
> print ("Locals:",locals())
>
> #foo("Monday")
>
> Does foo() have 1 or 2 locals?

foo has 2 local names. Its local namespace starts with 1 binding and may 
get another, depending of the value of the first.

If the size of the local namespace at some point of execution depends on 
unknown information, then of course the size at that point is also unknown.

Here is another example:

 >>> def f(a):
	print(locals())
	del a
	print(locals())
	a = None
	print(locals())
	
 >>> f(2)
{'a': 2}
{}
{'a': None}

-- 
Terry Jan Reedy