Why does inspect return different line for class inheriting from superclass?

2024/4/15 2:18:50

While trying to figure out if a function is called with the @decorator syntax, we realized that inspect has a different behaviour when looking at a decorated class that inherits from a superclass.

The following behaviour was found with CPython 3.6.2 under Windows 10.

It was also reproduced in CPython 3.7.0 under Linux 64 bits.

import inspectdef decorate(f):lines = inspect.stack()[1].code_contextprint(f.__name__, lines)return f@decorate
class Foo:pass@decorate
class Bar(dict):pass

Output

Foo ['@decorate\n']
Bar ['class Bar(dict):\n']

Why does inheritance change the behaviour of inspect?

Answer

Further experiment shows that this is a quirk of Python's line number assignment. Particularly, if we use dis to see the disassembly of code with and without a base class:

import dis
import sysdis.dis(sys._getframe().f_code)def dec(): pass@dec
class Foo: pass@dec
class Bar(Foo): pass

We see that for Foo, the instructions involved have line number 8 (corresponding to the @dec line):

  8          58 LOAD_NAME                4 (dec)61 LOAD_BUILD_CLASS62 LOAD_CONST               4 (<code object Foo at 0x2b2a65422810, file "./prog.py", line 8>)65 LOAD_CONST               5 ('Foo')68 MAKE_FUNCTION            071 LOAD_CONST               5 ('Foo')74 CALL_FUNCTION            2 (2 positional, 0 keyword pair)77 CALL_FUNCTION            1 (1 positional, 0 keyword pair)80 STORE_NAME               5 (Foo)

But for Bar, the line number advances from 11 to 12 for the LOAD_NAME that loads the base class:

 11          83 LOAD_NAME                4 (dec)86 LOAD_BUILD_CLASS87 LOAD_CONST               6 (<code object Bar at 0x2b2a654a0f60, file "./prog.py", line 11>)90 LOAD_CONST               7 ('Bar')93 MAKE_FUNCTION            096 LOAD_CONST               7 ('Bar')12          99 LOAD_NAME                5 (Foo)102 CALL_FUNCTION            3 (3 positional, 0 keyword pair)105 CALL_FUNCTION            1 (1 positional, 0 keyword pair)108 STORE_NAME               6 (Bar)

With no base class, the parent frame's f_lineno is on the @ line when the decorator runs. With a base class, the parent frame is on the load-the-base-class line.

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