DocTests for functools.cache()-decorated functions have no line number

[dlax]

Bug report

Bug description:

(edited to reflect discussion comments)

doctests of functions decorated with functools.cache (or lru_cache()) do not get their line number retrieved, e.g.:

# file /tmp/t.py
import functools

@functools.cache
def f(x):
 """cached

 >>> f(1)
 -2
 """
 return -x

$ python -m doctest /tmp/t.py
**********************************************************************
File "/tmp/t.py", line ?, in t.f
Failed example:
 f(1)
Expected:
 -2
Got:
 -1
**********************************************************************
1 item had failures:
 1 of 1 in t.f
***Test Failed*** 1 failure.

where we can see that the File "..." line is missing the line number of the example.
Also:

>>> import doctest
>>> import t
>>> dt, = doctest.DocTestFinder().find(t)
>>> dt
<DocTest t.f from /home/denis/src/cpython/t.py:None (1 example)>
>>> print(dt.lineno)
None

This is because DocTest._find_lineno() relies on inspect.isfunction() to possibly inspect function's code and get line numbers; but inspect.isfunction() returns False for a function decorated with functools.cache because only plain types.FunctionType is considered.

Original question: Should such cached functions be considered as well in inspect.isfunction()?

I would be happy to work on a fix in case this change is acceptable.

CPython versions tested on:

CPython main branch, 3.13

Operating systems tested on:

No response

Linked PRs

• gh-136914: Use inspect.isroutine() in DocTest's lineno computation #136930
• [3.13] gh-136914: Fix support of cached functions and properties in DocTest's lineno computation (GH-136930) #137615
• [3.14] gh-136914: Fix support of cached functions and properties in DocTest's lineno computation (GH-136930) #137616

[picnixz]

inspect.isfunction has very strict constraints and after being decorated by functools.cache, the object is no longer a function at runtime. So I don't think we should change anything in inspect. However we should change something in doctest.

EDIT: Also, it might not necessarily be the case that the decorator is called at the same place where the function has been defined (sometimes, people write it in two steps).

cc @serhiy-storchaka

[brianschubert]

For context, the was foreseen as being a possible issue in #115440 (review), but at the time it wasn't known whether a class wrapper using functools.wraps/functools.update_wrapper was a realistic use case. Evidently it is :-)

[serhiy-storchaka]

inspect.isroutine() is already used in some places of doctest. It should be used instead of inspect.isfunction() in other places as well.