[Python-Dev] Call for prudence about PEP-572 (original) (raw)

Giampaolo Rodola' g.rodola at gmail.com
Sat Jul 7 09:09:54 EDT 2018

Sorry in advance for opening yet another topic about PEP-572. With PEP-572 being officially accepted I know debating its inclusion in the language is a useless exercise at this point, but since it's still in "draft" status I would like to express my opinion as I think this is a feature which can potentially be abused fairly easily. FWIW I initially found myself disliking the idea as a whole but https://github.com/python/cpython/pull/8122 made me (and others) reconsider it quite a bit (see: https://twitter.com/grodola/status/1015251302350245888). PR-8122 clearly shows an improvement in expressiveness and compactness (many folks argue this is too much) but PEP-572 as it currently stands is too permissive IMHO. My concern about "easily abusable ugly cases" still remains, and I think they should be banned instead of just discouraged in the PEP or in the doc. Since we spend way more time reading code rather than writing it, as a "reader" I would expect a more prudent approach to the problem.

Proposal

  1. allow only one := assignment per line in "if" statements:

    if x := val1 and y := val2: # SyntaxError or SyntaxWarning if x == val1 and y := val2: # SyntaxError or SyntaxWarning if x := val1 and y == val2: # SyntaxError or SyntaxWarning if x := val1: # OK if (x := val1): # OK

  2. allow := in "while" statements, "if" statements and comprehensions only:

    foo(x := 0) # SyntaxError yield x := 3 # SyntaxError assert y := 3 # SyntaxError

  3. (debatable) disallow := if the variable is already defined:

    x = 5 if (x := val): # SyntaxError or SyntaxWarning

  4. ban "a = (b := c)", "x = a := (b := (c := d))" and similar (they're just too ugly IMHO)

Rationale 1

In visual terms assignments in Python have always occurred at the BEGINNING of the line and always on the most LEFT side:

>>> foo = fun1()
>>> bar = fun2()
>>> baz = fun3()

That is where I naturally expect an assignment to be when reading code. My main concern with PEP-572 is that an assignments can now occur at any point in the line:

>>> foo = fun1()
>>> bar = fun2()
>>> if foo == val1 and bar == val2 and baz := fun3():
...    ...

That forces me to visually scan the whole line horizontally from left to right 'till its end, looking for possible variables being set. I'm concerned that I will miss := occurrences because visually they are very similar to == unless parentheses are made mandatory:

>>> if foo == val1 and bar == val2 and (baz := fun3()):
...    ...

Also, in case of multi-line conditionals I have to visually scan the construct both horizontally AND vertically:

>>> if (foo == val1 and \
...        bar == val2 and \
...        baz := val3):
...     ...

Again, that is not a place where I would expect to find or look for a variable assignment. I know I wouldn't like to read or review a code which does that and I suspect linters will likely end up wanting to emit a warning in that case (see: https://github.com/PyCQA/pylint/issues/2246). https://github.com/python/cpython/pull/8116/files avoids using multiple := per line and that's why the result appears readable enough IMO.

Rationale 2

PEP-572 states:

> The := operator may be used directly in a positional function call

argument

That means allowing:

>>> foo(x := 0)

I honestly don't see why anyone would want to call a function AND assign a variable value at the same time (except in comprehensions). With this in place I not only have to guard against "if" statements assigning values at any point in the code, but also function calls, both horizontally and vertically e.g.:

>>> foo(some_long_var_name, another_one, x := bar(),
        y := fun())

To me this looks like the perfect example of where this functionality can be abused. Also, I'm not clear what PEP-572 intend to do about "all other places". E.g. should these cases be allowed? (IMO no)

>>> yield x := 3
>>> assert y := 3

-- Giampaolo - http://grodola.blogspot.com -------------- next part -------------- An HTML attachment was scrubbed... URL: <http://mail.python.org/pipermail/python-dev/attachments/20180707/54793a72/attachment.html>

More information about the Python-Dev mailing list