Python: move unary operator spacing into SpacesVisitor

[knutwannheden]

Motivation

• Python: MinimumViableSpacing to add spaces before not #8540 taught MinimumViableSpacingVisitor to put a space between not and its operand whenever the operand's prefix was empty. That space is a style decision rather than a syntactic requirement — not(x), not[1] and not'x' are all legal Python — and MinimumViableSpacingVisitor runs over the whole compilation unit inside AutoFormat, so auto-formatting rewrote not(x) to not (x) in files no recipe had touched, with nothing downstream reconciling it.

The Java formatter already draws this line: AutoFormatVisitor runs MinimumViableSpacingVisitor and then SpacesVisitor, and SpacesVisitor.visitUnary normalizes the operand prefix per style. Python had neither half — SpacesVisitor has no visitUnary, so whatever the minimum-spacing pass produced was final.

The same pass was also missing the other side of the problem. A recipe that wraps an assert condition in not without transferring the condition's prefix printed assertnot x, and auto-format left it that way.

Examples

SpacesVisitor now owns unary spacing, which for Python means not always gets a separator and +, - and ~ are tightened — what PEP 8, black and PyCharm all produce:

assert not(x)   # -> assert not (x)
y = - z         # -> y = -z

MinimumViableSpacingVisitor adds a space only where a keyword and the expression that follows it would lex as a single token, so on its own it leaves both of those alone. Given a recipe that builds not with empty prefixes, it now produces:

assert x        # -> assert not x
assert (x)      # -> assert not(x)
assert 'x'      # -> assert not'x'
assert f'{x}'   # -> assert not f'{x}'

The last two differ because the f-string's source starts with an identifier character.

Summary

• SpacesVisitor.visit_unary normalizes the operand prefix: one space after not, none after +, - and ~. No new SpacesStyle field — Python's SpacesStyle.AroundOperators mirrors PyCharm's Python settings, which expose no unary option, and and/or already hardcode their separator the same way.
• MinimumViableSpacingVisitor decides by printing the expression and testing its first character, so the rule tracks what is actually emitted rather than a list of node types. The empty-prefix guard runs first, so parsed code essentially never reaches the print call.
• That rule is applied to the expression following assert, return, and/or, in/not in/is/is not, and both the if and else of a conditional expression.

if and while conditions need nothing: they are wrapped in ControlParentheses, which keeps the space after the keyword when a recipe replaces the inner tree.

Two things are deliberately left out. The mirror direction — an expression immediately before a keyword, e.g. a if c if a recipe cleared padding.true_part.before — needs the last printed character of the left side and has no failing case yet. So do the remaining keyword parents (del, raise, yield, await, lambda, global, import/as), each a few lines with the helper now in place.

Test plan

• tests/python/all/format/spaces_test.py — new file covering not gaining a space, collapsing not x, and tightening - z and ~ z
• tests/python/all/format/minimum_viable_spacing_test.py — not(x) and not[1] unchanged; generated not separated from an identifier and an f-string but not from (, [ or a string literal; generated operands separated from assert, return, and, in and the if/else of a conditional expression
• Each new test verified to fail before the corresponding change
• pytest tests --ignore=tests/rpc — 1933 passed, 61 skipped