Python 3.8 assignment expressions – first encounter

python-3.8-assignment-expressions.md

Python 3.8 vs PEP 572

According to PEP 572, section Scope of the target, the following examples of assignment expressions should be invalid and raise TargetScopeError.

Example 1

[i := i+1 for i in range(5)]

Python 3.8.0a1 (tags/v3.8.0a1:e75eeb00b5, Feb  3 2019, 20:47:39) [MSC v.1916 64 bit (AMD64)] on win32
>>> test = [i := i+1 for i in range(5)]
>>> i
5
>>> test
[1, 2, 3, 4, 5]
>>>

No error here. Works as expected, equivalent to

>>> test = []
>>> for i in range(5):
...     i = i + 1
...     test.append(i)
...
>>> i
5
>>> test
[1, 2, 3, 4, 5]
>>>

Example 2

[[(j := j) for i in range(5)] for j in range(5)]

Python 3.8.0a1 (tags/v3.8.0a1:e75eeb00b5, Feb  3 2019, 20:47:39) [MSC v.1916 64 bit (AMD64)] on win32
>>> test = [[(j := j) for i in range(5)] for j in range(5)]
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "<stdin>", line 1, in <listcomp>
  File "<stdin>", line 1, in <listcomp>
NameError: name 'j' is not defined
>>>

I don't understand this error. This listcomp should be equivalent to

>>> test = []
>>> for j in range(5):
...     sublist = []
...     for i in range(5):
...         j = j
...         sublist.append(j)
...     test.append(sublist)
...
>>> j
4
>>> test
[[0, 0, 0, 0, 0], [1, 1, 1, 1, 1], [2, 2, 2, 2, 2], [3, 3, 3, 3, 3], [4, 4, 4, 4, 4]]
>>>

Or, according to PEP 572, Appendix B, equivalent to

>>> def f():
...     if False:
...         j = None
...     def outer_generator(outer_iterator):
...         nonlocal j
...         def inner_generator(inner_iterator):
...             nonlocal j
...             for i in inner_iterator:
...                 j = j
...                 yield j
...         for j in outer_iterator:
...             yield list(inner_generator(range(5)))
...     test = list(outer_generator(range(5)))
...     print(j)
...     print(test)
...
>>> f()
4
[[0, 0, 0, 0, 0], [1, 1, 1, 1, 1], [2, 2, 2, 2, 2], [3, 3, 3, 3, 3], [4, 4, 4, 4, 4]]
>>>

Example 3

[i := 0 for i, j in stuff]

Python 3.8.0a1 (tags/v3.8.0a1:e75eeb00b5, Feb  3 2019, 20:47:39) [MSC v.1916 64 bit (AMD64)] on win32
>>> stuff = [(11, 22), (33, 44)]
>>> test = [i := 0 for i, j in stuff]
>>> i
0
>>> test
[0, 0]
>>>

No error here. Works as expected, equivalent to

>>> stuff = [(11, 22), (33, 44)]
>>> test = []
>>> for i, j in stuff:
...     i = 0
...     test.append(i)
...
>>> i
0
>>> test
[0, 0]
>>>

Example 4

[i+1 for i in i := stuff]

Python 3.8.0a1 (tags/v3.8.0a1:e75eeb00b5, Feb  3 2019, 20:47:39) [MSC v.1916 64 bit (AMD64)] on win32
>>> stuff = [11, 22, 33, 44]
>>> test = [i+1 for i in i := stuff]
  File "<stdin>", line 1
    test = [i+1 for i in i := stuff]
                           ^
SyntaxError: invalid syntax

>>> test = [i+1 for i in (i := stuff)]
>>> i
[11, 22, 33, 44]
>>> test
[12, 23, 34, 45]
>>>

Obvious syntax error (in has higher priority than :=). After adding parentheses, the listcomp works as expected, equivalent to

>>> stuff = [11, 22, 33, 44]
>>> test = []
>>> i = stuff
>>> for i in i:
...     test.append(i+1)
...
>>> i
44
>>> test
[12, 23, 34, 45]
>>>

(except that in the latter case, the loop target i is not hidden in an implicit inner scope)

Update 26 Feb 2019:

The above tested behavior is the same in version 3.8.0a2.

Update 1 Apr 2019:

The above tested behavior is the same in version 3.8.0a3.

Update 17 May 2019:

The above tested behavior is the same in version 3.8.0a4.

Update 5 Jun 2019:

The above tested behavior is the same in version 3.8.0b1.

Update 11 Jul 2019:

The above tested behavior is the same in version 3.8.0b2.

Update 8 Aug 2019:

The above tested behavior is the same in version 3.8.0b3.

Update 18 Sep 2019:

The above tested behavior changed in version 3.8.0b4. All the example comprehensions now raise SyntaxError:

Python 3.8.0b4 (tags/v3.8.0b4:d93605d, Aug 29 2019, 23:21:28) [MSC v.1916 64 bit (AMD64)] on win32
>>> test = [i := i+1 for i in range(5)]
  File "<stdin>", line 1
SyntaxError: assignment expression cannot rebind comprehension iteration variable 'i'
>>>

Python 3.8.0b4 (tags/v3.8.0b4:d93605d, Aug 29 2019, 23:21:28) [MSC v.1916 64 bit (AMD64)] on win32
>>> test = [[(j := j) for i in range(5)] for j in range(5)]
  File "<stdin>", line 1
SyntaxError: assignment expression cannot rebind comprehension iteration variable 'j'
>>>

Python 3.8.0b4 (tags/v3.8.0b4:d93605d, Aug 29 2019, 23:21:28) [MSC v.1916 64 bit (AMD64)] on win32
>>> stuff = [(11, 22), (33, 44)]
>>> test = [i := 0 for i, j in stuff]
  File "<stdin>", line 1
SyntaxError: assignment expression cannot rebind comprehension iteration variable 'i'
>>>

Python 3.8.0b4 (tags/v3.8.0b4:d93605d, Aug 29 2019, 23:21:28) [MSC v.1916 64 bit (AMD64)] on win32
>>> stuff = [11, 22, 33, 44]
>>> test = [i+1 for i in (i := stuff)]
  File "<stdin>", line 1
SyntaxError: assignment expression cannot be used in a comprehension iterable expression
>>>

Update 2 Oct 2019:

No change between version 3.8.0b4 and 3.8.0rc1.

Update 21 Oct 2019:

No change between version 3.8.0rc1 and 3.8.0.

Note:

No TargetScopeError or any other new exception class has been added in Python 3.8.