python - List comprehension rebinds names even after scope of comprehension. Is this right?

121
votes

Comprehensions are having some unexpected interactions with scoping. Is this the expected behavior?

I've got a method:

def leave_room(self, uid):
  u = self.user_by_id(uid)
  r = self.rooms[u.rid]

  other_uids = [ouid for ouid in r.users_by_id.keys() if ouid != u.uid]
  other_us = [self.user_by_id(uid) for uid in other_uids]

  r.remove_user(uid) # OOPS! uid has been re-bound by the list comprehension above

  # Interestingly, it's rebound to the last uid in the list, so the error only shows
  # up when len > 1

At the risk of whining, this is a brutal source of errors. As I write new code, I just occasionally find very weird errors due to rebinding -- even now that I know it's a problem. I need to make a rule like "always preface temp vars in list comprehensions with underscore", but even that's not fool-proof.

The fact that there's this random time-bomb waiting kind of negates all the nice "ease of use" of list comprehensions.

6
pythonbindinglist-comprehension
-1: "brutal source of errors"? Hardly. Why choose such an argumentative term? Generally the most expensive errors are requirements misunderstandings and simple logic errors. This kind of error has been a standard problem in a lot of programming languages. Why call it 'brutal'? - S.Lott
It violates the principle of least surprise. It's also not mentioned in the python documentation on list comprehensions which does however mention several times how easy and convenient they are. Essentially it's a land-mine that existed outside my language model, and hence was impossible for me to foresee. - Jabavu Adams
+1 for "brutal source of errors". The word 'brutal' is entirely justified. - Nathaniel
The only "brutal" thing I see here is your naming convention. This isn't the 80s any more you're not limited to 3 character variable names. - UloPe
Note: the documention does state that list-comprehension are equivalent to the explicit for-loop construct and for-loops leak variables. So it wasn't explicit but was implicitly stated. - Bakuriu

6 Answers

179
votes

List comprehensions leak the loop control variable in Python 2 but not in Python 3. Here's Guido van Rossum (creator of Python) explaining the history behind this:

We also made another change in Python 3, to improve equivalence between list comprehensions and generator expressions. In Python 2, the list comprehension "leaks" the loop control variable into the surrounding scope:

x = 'before'
a = [x for x in 1, 2, 3]
print x # this prints '3', not 'before'

This was an artifact of the original implementation of list comprehensions; it was one of Python's "dirty little secrets" for years. It started out as an intentional compromise to make list comprehensions blindingly fast, and while it was not a common pitfall for beginners, it definitely stung people occasionally. For generator expressions we could not do this. Generator expressions are implemented using generators, whose execution requires a separate execution frame. Thus, generator expressions (especially if they iterate over a short sequence) were less efficient than list comprehensions.

However, in Python 3, we decided to fix the "dirty little secret" of list comprehensions by using the same implementation strategy as for generator expressions. Thus, in Python 3, the above example (after modification to use print(x) :-) will print 'before', proving that the 'x' in the list comprehension temporarily shadows but does not override the 'x' in the surrounding scope.

49
votes

Yes, list comprehensions "leak" their variable in Python 2.x, just like for loops.

In retrospect, this was recognized to be a mistake, and it was avoided with generator expressions. EDIT: As Matt B. notes it was also avoided when set and dictionary comprehension syntaxes were backported from Python 3.

List comprehensions' behavior had to be left as it is in Python 2, but it's fully fixed in Python 3.

This means that in all of:

list(x for x in a if x>32)
set(x//4 for x in a if x>32)         # just another generator exp.
dict((x, x//16) for x in a if x>32)  # yet another generator exp.
{x//4 for x in a if x>32}            # 2.7+ syntax
{x: x//16 for x in a if x>32}        # 2.7+ syntax

the x is always local to the expression while these:

[x for x in a if x>32]
set([x//4 for x in a if x>32])         # just another list comp.
dict([(x, x//16) for x in a if x>32])  # yet another list comp.

in Python 2.x all leak the x variable to the surrounding scope.

UPDATE for Python 3.8(?): PEP 572 will introduce := assignment operator that deliberately leaks out of comprehensions and generator expressions! It's motivated by essentially 2 use cases: capturing a "witness" from early-terminating functions like any() and all():

if any((comment := line).startswith('#') for line in lines):
    print("First comment:", comment)
else:
    print("There are no comments")

and updating mutable state:

total = 0
partial_sums = [total := total + v for v in values]

See Appendix B for exact scoping. The variable is assigned in closest surrounding def or lambda, unless that function declares it nonlocal or global.

8
votes

Yes, assignment occurs there, just like it would in a for loop. No new scope is being created.

This is definitely the expected behavior: on each cycle, the value is bound to the name you specify. For instance,

>>> x=0
>>> a=[1,54,4,2,32,234,5234,]
>>> [x for x in a if x>32]
[54, 234, 5234]
>>> x
5234

Once that's recognized, it seems easy enough to avoid: don't use existing names for the variables within comprehensions.

2
votes

Interestingly this doesn't affect dictionary or set comprehensions.

>>> [x for x in range(1, 10)]
[1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> x
9
>>> {x for x in range(1, 5)}
set([1, 2, 3, 4])
>>> x
9
>>> {x:x for x in range(1, 100)}
{1: 1, 2: 2, 3: 3, 4: 4, 5: 5, 6: 6, 7: 7, 8: 8, 9: 9, 10: 10, 11: 11, 12: 12, 13: 13, 14: 14, 15: 15, 16: 16, 17: 17, 18: 18, 19: 19, 20: 20, 21: 21, 22: 22, 23: 23, 24: 24, 25: 25, 26: 26, 27: 27, 28: 28, 29: 29, 30: 30, 31: 31, 32: 32, 33: 33, 34: 34, 35: 35, 36: 36, 37: 37, 38: 38, 39: 39, 40: 40, 41: 41, 42: 42, 43: 43, 44: 44, 45: 45, 46: 46, 47: 47, 48: 48, 49: 49, 50: 50, 51: 51, 52: 52, 53: 53, 54: 54, 55: 55, 56: 56, 57: 57, 58: 58, 59: 59, 60: 60, 61: 61, 62: 62, 63: 63, 64: 64, 65: 65, 66: 66, 67: 67, 68: 68, 69: 69, 70: 70, 71: 71, 72: 72, 73: 73, 74: 74, 75: 75, 76: 76, 77: 77, 78: 78, 79: 79, 80: 80, 81: 81, 82: 82, 83: 83, 84: 84, 85: 85, 86: 86, 87: 87, 88: 88, 89: 89, 90: 90, 91: 91, 92: 92, 93: 93, 94: 94, 95: 95, 96: 96, 97: 97, 98: 98, 99: 99}
>>> x
9

However it has been fixed in 3 as noted above.

1
votes

some workaround, for python 2.6, when this behaviour is not desirable

# python
Python 2.6.6 (r266:84292, Aug  9 2016, 06:11:56)
Type "help", "copyright", "credits" or "license" for more information.
>>> x=0
>>> a=list(x for x in xrange(9))
>>> x
0
>>> a=[x for x in xrange(9)]
>>> x
8
-1
votes

In python3 while in list comprehension the variable is not getting change after it's scope over but when we use simple for-loop the variable is getting reassigned out of scope.

i = 1 print(i) print([i in range(5)]) print(i) Value of i will remain 1 only.

Now just use simply for loop the value of i will be reassigned.