Issue 17960: Clarify the required behaviour of locals() (original) (raw)

Since "soonish" turned out to be "4 years and counting", copying in the specifics of the proposal in from the old python-dev thread:

1. While nominally undefined, in practice lots of Python programs depend on the locals() builtin behaving exactly how it behaves in CPython.

2. PyPy at least has replicated that behaviour faithfully (to the extent of replicating our weird trace function related misbehaviour, as recently pointed out in issue #30744)

3. For module scopes and class scopes (and the corresponding forms of exec and eval), the expected behaviour is relatively straightforward to both define and implement:

• at module scope, as well as when using exec() or eval() with a single namespace, locals() must return the same thing as globals(), which must be the actual execution namespace. Subsequent execution may change the contents of the returned mapping, and changes to the returned mapping must change the execution environment.

• at class scope, as well as when using exec() or eval() with separate global and local namespaces, locals() must return the specified local namespace (which may be supplied by the metaclass prepare method in the case of classes). Subsequent execution may change the contents of the returned mapping, and changes to the returned mapping must change the execution environment. For classes, this mapping will not be used as the actual class namespace underlying the defined class (the class creation process will copy the contents to a fresh dictionary that is only accessible by going through the class machinery).

1. For function scopes, the appropriate semantics are less clear, as what CPython currently does is fairly weird and quirky.

• actual execution uses the fast locals array and cell references (for nonlocal variables)
• there's a PyFrame_FastToLocals operation that populates the frame's "f_locals" attribute based on the current state of the fast locals array and any referenced cells
• a direct reference to f_locals is returned from locals(), so if you hand out multiple concurrent references, then all those references will be to the exact same dictionary
• the two common calls to the reverse operation, PyFrame_LocalsToFast, were removed in the migration to Python 3: exec is no longer a statement and hence can longer affect function local namespaces, and the compiler now disallows the use of "from module import *" operations at function scope
• however, two obscure calling paths remain: PyFrame_LocalsToFast is called as part of returning from a trace function, and you can also still inject the IMPORT_STAR opcode when creating a function directly from a code object rather than via the compiler

It would be a lot simpler to document the expected behaviour at function scope if locals() were to be updated to return a true snapshot (i.e. a copy of f_locals, rather than a direct reference), with direct access to the shared locals reference requiring going through the frame attribute. That way, trace functions could still modify local variables (since they use frame.f_locals), but setting a trace function wouldn't suddenly have the side effect of making modifications to locals() take effect at function scope.

Status update: I've posted an initial PR to issue 30744 that relies on the trace hook semantic change proposed in the PEP to resolve the trace hook/cell reference incompatibility reported there.

That provides confidence that it really is only the semantics of trace hooks that we need to change, rather than anything about locals() or frame.f_locals in general.

So the next steps will be to do a final editing pass on the current PEP to account for the reference implementation, and then send it to python-dev for official review and pronouncement.