Improve handling of assignment expressions

[cdce8p]

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Description

Fixed some small issues with assignment expressions.

Multiline strings

l1 = f'The number {(count1 := 4)} ' \
     f'is equal to {count1}'

This caused a used-before-assignment error in Python 3.8. The root cause is in the builtin ast module. Until 3.9, all nodes where parsed as being on the same line. I added some special case handling for it.
However this might cause some errors to remain unnoticed!

Assignment expression with Expr

foo if (foo := 3 - 2) > 0 else 0

Assignment expression with AnnAssign and AugAssign

x2: bool = b2 if (b2 := True) else False
x3 = 0
x3 += b3 if (b3 := 4) else 6

--

I believe this could be included with 2.7.3

Type of Changes

	Type
✓	🐛 Bug fix

Related Issue

Closes #3763
Closes #4238

[coveralls]

Coverage increased (+0.001%) to 91.437% when pulling 6d6a326 on cdce8p:fix-used-before-assignment into cb3ea62 on PyCQA:master.

[Pierre-Sassoulas]

Looks reasonable. What kind of errors could go unnoticed ? Tests are still green which is a good sign 😄

[cdce8p]

What kind of errors could go unnoticed ? Tests are still green which is a good sign 😄

Could you explain what you mean? Without this change the newly added tests would create used-before-assignment errors.

[Pierre-Sassoulas]

Maybe I misunderstood your description I was referring to However this might cause some errors to remain unnoticed! ?

[cdce8p]

Yeah, take the following example:

l1 = f'The number {(count1 := 4)} ' \
     f'is equal to {count1}'

l2 = f'is equal to {count2}' \
     f'The number {(count2 := 4)} '

count2 is defined after it's used, so it won't work.

Without this change, both l1 and l2 would throw used-before-assignment errors.
With the fix, none will (in py38 and below). That's because both statements appear to be on the same line (with the old ast module). So there is no way to check if it's a valid use case or not.

[cdce8p]

Just FWI: This MR is would be ready for merge from my side.