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library symtable.SymbolTable.is_nested() description differs from Cpython implementation. #99350
Description
The documentation says
is_nested()
Return True if the block is a nested class or function.
The cpython implementation of symtable.c sets the corresponding flag only if the block is nested somewhere inside a function. This would include, for example, a scope inside a class inside a function.
Thus
def f(): # false
def g(): # true
def h(): # true
class C: # true
class B: # true
def h(): # true
class C: # true
class A: # false
def g(): # false !!
def h(): # true
class C: # true
class B: # false !!
def h(): # false !!
class C: # false !!The value is the same as the CO_NESTED bit in the code object's co_flags member.
The flag is also documented (similarly incorrectly) in the inspect module documentation.
I would recommend deprecating this feature. The documentation for co_flags says that
bits 0x10 and 0x1000 were used in earlier versions of Python.
0x10 is the CO_NESTED bit. I would guess that nobody uses CO_NESTED or SymbolTable.is_nested().
Alternatives:
- Change the behavior in Cpython to agree with the documentation. A class nested in another scope should also have CO_NESTED set. Make a test case for the compiler, to ensure that other implementations follow the same definition.
- Correct the documentation for symtable and inspect modules. Possibly also deprecate the feature.
Activity
MonadChains commentedon Nov 12, 2022
Discussion about possible deprecation of CO_NESTED: #96811
There is also a previous PR opened on the subject #19660
johnslavik commentedon Aug 17, 2025
Yes, the deprecation of
CO_NESTEDwas rejected in #96811.However, it would still be useful to update the docs of the flag as it does not only flag nested functions, but all function-like blocks only (that includes nested
typestatements, nested generator comprehensions, nested sets of type parameters to generic functions which "wrap" regular function code objects, set/list/dict comprehensions until 3.11 etc.)It's still true that
CO_NESTEDdoesn't mark nested classes; neither when they're nested in a function nor in another class.This prints:
{'<generic parameters of nested_generic>': 19, '<genexpr>': 51, 'NestedAnn': 19, 'NestedClass': 0, 'nested_func': 19}A documentation update would be great, I'll try to make one.
picnixz commentedon Aug 17, 2025
I think it would be just better to fix symtable actually. For introspection it could be the only way to do it properly and in a compatible manner, at least assuming CPython implementation. The rationale for rejecting the deprecation is because it is useful for others.