Add support for `frozendict` as AST `Constant`

[lgeiger]

Currently the AST doesn't allow frozendict as constants when compiled:

import ast
node = ast.Expression(body=ast.Constant(value=frozendict({"a": 1})))
compile(ast.fix_missing_locations(node), "<string>", "eval")

Traceback (most recent call last):
  File "<python-input-0>", line 4, in <module>
    compile(ast.fix_missing_locations(node), "<string>", "eval")
    ~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: got an invalid type in Constant: frozendict

This is implemented for frozenset

node = ast.Expression(body=ast.Constant(value=frozenset({"a", "b"})))
compile(ast.fix_missing_locations(node), "<string>", "eval")

<code object <module> at 0x1067203b0, file "<string>", line 1>

so it makes sense to do the same for frozendict.

Linked PRs

• gh-150045: Add frozendict support to the compiler and AST constants #150050

[JelleZijlstra]

Can you clarify what you need? This already works:

>>> import ast
>>> ast.Constant(frozendict({"a": 1, "b": 1}))
... 
Constant(value=frozendict({'a': 1, 'b': 1}), kind=None)
>>> ast.unparse(_)
"frozendict({'a': 1, 'b': 1})"

And I see no special casing for frozenset in the AST module.

[lgeiger]

@JelleZijlstra sorry for the incomplete issue. I hit cmd-enter accidentally but you where faster than I could fix it 🚀.

Indeed defining a frozendict constant works. However it fails when compiling:

import ast
node = ast.Expression(body=ast.Constant(value=frozendict({"a": 1})))
compile(ast.fix_missing_locations(node), "<string>", "eval")

I updated the issue comment. I am happy to submit a PR to fix this. I think I have it mostly working.

[methane]

In the case of frozenset, we use frozenset to optimize code like if x in {"spam", "ham", "egg"}.

But would this kind of optimization really be useful for frozendict?
Or is there some other reason for frozendict to be supported by the AST?

I am opposed to adding features that are not used just for the sake of symmetry with frozenset.