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

Question

Why the result of below snippets are different?

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',)

As you see, the results are different and I don't know why they are different.

Answer 1

Ok, this is a bit subtle. While co_varnames should give you all local variables, in this case,i is actually a free variable in Bar, so it is in Bar's closure.

Notice what happens when we dissasemble the bytecode:

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

Notice, the opcode is STORE_DEREF not STORE_FAST. So, consider:

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>,)

Since it is referenced in a nested function, this will be available in co_cellvars:

In [12]: foo_code = Foo.__code__

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

This is explained in the data-model documentation for internal types:

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...

So to get all local variables you need:

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