Is it possible to insert extra scopes in python

Question

I would like to know if it is possible to inset an extra scope within my code, without defining a function. The problem that I had was as follows:

def func(a, b, c):
   some_var = [a for a in b]
   # do stuff with a

My expectation was that a is still the value that I passed to the function, since I expected the other a to be in the scope of the list comprehension (had to search for that bug for ever!).

How can I rewrite it so that the a in the list comprehension is in its own scope?

I would like to have something more elegant than

def func(a, b, c):
   def helper(v): 
       return [a for a in v] 
   some_var = helper(b)
   # do stuff with a

In languages where the scope is defined by using {}, you could do stuff like

def func(a, b, c) {
   some_var = null
   {
       some_var = [a for a in b]
   }
   # do stuff with a
}

Is there something similart in Python?

what python version is this? and why arent you on python 3 already? — Paritosh Singh, Jan 11 '19 at 16:17
List comprehension (but not set comprehension, dict comprehensions, nor generator expressions) have leaky scope in Python 2. This was fixed in Python 3 — juanpa.arrivillaga, Jan 11 '19 at 16:19
@ParitoshSingh Python 2.7. It is a requirement from my work (I know, I know, also like python 3 more) — User12547645, Jan 11 '19 at 16:19
@juanpa.arrivillaga So this problem will no longer occure in Python 3? — User12547645, Jan 11 '19 at 16:20
Thank you guys. Please just write one answer to this. I will then close the question — User12547645, Jan 11 '19 at 16:21
@User12547645 can't you just rename the inner variable to something else? — Ralf, Jan 11 '19 at 16:22
@User12547645 Just rename the loop variable? If you want it to be in its own scope the name doesn't matter anyway. — a_guest, Jan 11 '19 at 16:22
@Ralf You are right. But emagine you have a more coplex function with a lot of different varable. I don`t want to check if a variable was used already. I just want to define an extra scope — User12547645, Jan 11 '19 at 16:23

score 2 · Answer 1 · answered Jan 11 '19 at 16:23

If you can, move to python 3. List comprehensions in python 2 are known to leak variables into the namespace.

If not however, you can mimic a clean list comprehension by using generator expressions which do not leak variables, wrapped in a list call.

def func(a, b, c):
   some_var = list(a for a in b) #Does not leak a
   # do stuff with a

score 2 · Answer 2 · answered Jan 11 '19 at 16:24

2

You can insert an extra scope by wrapping the list comprehension by a dict comprehension for example:

>>> {None: [x for x in range(5)] for __ in range(1)}.values()[0]

This won't leak the value of x but it's not really readable.

answered Jan 11 '19 at 16:24

a_guest

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score 0 · Answer 3 · answered Jan 11 '19 at 16:29

0

If you are just creating a copy of b, isn't it easier to just do:

def func(a, b, c):
   some_var = b[:]

No need to worry about a being in an extra scope and what not.

answered Jan 11 '19 at 16:29

r.ook

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Is it possible to insert extra scopes in python

3 Answers3