Assignment expression in assert causes SyntaxError

[ghost]

BPO	39909
Nosy	@vstinner, @ericvsmith

^{Note: these values reflect the state of the issue at the time it was migrated and might not reflect the current state.}

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GitHub fields:

assignee = None
closed_at = <Date 2020-03-09.08:25:00.808>
created_at = <Date 2020-03-09.07:26:04.755>
labels = ['interpreter-core', 'invalid', 'type-bug', '3.8']
title = 'Assignment expression in assert causes SyntaxError'
updated_at = <Date 2020-03-09.08:25:09.449>
user = None

bugs.python.org fields:

activity = <Date 2020-03-09.08:25:09.449>
actor = '\xd0\x9c\xd0\xb8\xd1\x85\xd0\xb0\xd0\xb8\xd0\xbb \xd0\x9a\xd1\x8b\xd1\x88\xd1\x82\xd1\x8b\xd0\xbc\xd0\xbe\xd0\xb2'
assignee = 'none'
closed = True
closed_date = <Date 2020-03-09.08:25:00.808>
closer = 'eric.smith'
components = ['Interpreter Core']
creation = <Date 2020-03-09.07:26:04.755>
creator = '\xd0\x9c\xd0\xb8\xd1\x85\xd0\xb0\xd0\xb8\xd0\xbb \xd0\x9a\xd1\x8b\xd1\x88\xd1\x82\xd1\x8b\xd0\xbc\xd0\xbe\xd0\xb2'
dependencies = []
files = []
hgrepos = []
issue_num = 39909
keywords = []
message_count = 6.0
messages = ['363698', '363699', '363700', '363701', '363702', '363703']
nosy_count = 3.0
nosy_names = ['vstinner', 'eric.smith', '\xd0\x9c\xd0\xb8\xd1\x85\xd0\xb0\xd0\xb8\xd0\xbb \xd0\x9a\xd1\x8b\xd1\x88\xd1\x82\xd1\x8b\xd0\xbc\xd0\xbe\xd0\xb2']
pr_nums = []
priority = 'normal'
resolution = 'not a bug'
stage = 'resolved'
status = 'closed'
superseder = None
type = 'behavior'
url = 'https://bugs.python.org/issue39909'
versions = ['Python 3.8']

[ghost]

Assignment expression in assert causes SyntaxError

Minimal case:

assert var := None

Error:

  File "<input>", line 1
    assert var := None
               ^
SyntaxError: invalid syntax

Workaround:

assert (var := None)

My use case:

my_dict = dict()
assert value := my_dict.get('key')

[ericvsmith]

I believe this is by design, as is the same restriction on statement-level assignment expressions. And if requiring parens discourages the use of assignment expressions in asserts, I think that's a good thing.

Why not just use the workaround you've already identified?

[ghost]

if this is by design i'm fine with somebody closing this issue.

i've not found mention of such case in pep so i though it's not by design.
docs say assert just takes "expression" and "assignment expression" has "expression" in it's name.
https://docs.python.org/3/reference/simple_stmts.html#the-assert-statement

workaraund does not look as cool because of parentheses.

again, if this is "as intended" i'm fine with closing issue.

[vstinner]

assert var := None

It's good that this syntax is rejected: it looks like a typo (assert var == None). Moreover, it's part of of the PEP-572 design.
https://www.python.org/dev/peps/pep-0572/#exceptional-cases

Python works as expected, I suggest to close the issue as not a bug.

[ericvsmith]

I'm going to close this, since I can't imagine this restriction being relaxed.

[ghost]

ok