为什么 co_varnames return 不列出所有变量名？

Question

为什么下面代码片段的结果不同？

def Foo():
  i = 0
  def Bar():
    nonlocal i
    i = 1
  return Bar()

print(Foo.__code__.co_varnames)

# it will print: ('Bar',)

def Foo():
  i = 0
  def Bar():
    i = 1
  return Bar()

print(Foo.__code__.co_varnames)

# it will print: ('i', 'Bar',)

如你所见，结果不一样，不知道为什么不一样。

Answer 1

好的，这有点微妙。虽然 co_varnames 应该给你所有 local 变量，在这种情况下，i 实际上是 Bar 中的一个自由变量，所以它在 Bar的闭包.

注意当我们分解字节码时会发生什么：

In [2]: def Foo():
   ...:   i = 0
   ...:   def Bar():
   ...:     nonlocal i
   ...:     i = 1
   ...:   return Bar()
   ...:

In [3]: Foo.__code__.co_varnames
Out[3]: ('Bar',)


In [4]: import dis

In [5]: dis.dis(Foo)
  2           0 LOAD_CONST               1 (0)
              2 STORE_DEREF              0 (i)

  3           4 LOAD_CLOSURE             0 (i)
              6 BUILD_TUPLE              1
              8 LOAD_CONST               2 (<code object Bar at 0x101072be0, file "<ipython-input-4-a3a062461e32>", line 3>)
             10 LOAD_CONST               3 ('Foo.<locals>.Bar')
             12 MAKE_FUNCTION            8 (closure)
             14 STORE_FAST               0 (Bar)

  6          16 LOAD_FAST                0 (Bar)
             18 CALL_FUNCTION            0
             20 RETURN_VALUE

Disassembly of <code object Bar at 0x101072be0, file "<ipython-input-4-a3a062461e32>", line 3>:
  5           0 LOAD_CONST               1 (1)
              2 STORE_DEREF              0 (i)
              4 LOAD_CONST               0 (None)
              6 RETURN_VALUE

注意，操作码是 STORE_DEREF 而不是 STORE_FAST。所以，考虑：

In [9]: def Foo():
   ...:   i = 0
   ...:   def Bar():
   ...:     nonlocal i
   ...:     i = 1
   ...:   return Bar
   ...:

In [10]: bar = Foo()

In [11]: bar.__closure__
Out[11]: (<cell at 0x1040a7940: int object at 0x100980bc0>,)

由于它在嵌套函数中被引用，这将在 co_cellvars:

中可用

In [12]: foo_code = Foo.__code__

In [13]: foo_code.co_cellvars
Out[13]: ('i',)

这在内部类型的数据模型 documentation 中有解释：

co_varnames is a tuple containing the names of the local variables (starting with the argument names); co_cellvars is a tuple containing the names of local variables that are referenced by nested functions; co_freevars is a tuple containing the names of free variables...

所以要获得所有局部变量你需要：

In [16]: Foo.__code__.co_varnames + Foo.__code__.co_cellvars
Out[16]: ('Bar', 'i')

为什么 co_varnames return 不列出所有变量名？

Why doesn't co_varnames return list of all the variable names?

python

python-3.x

python-internals