Why generate POP_TOP after an "import from?"
Alexandre Brault
abrault at mapgears.com
Fri Apr 17 14:37:38 EDT 2020
On 2020-04-17 2:22 p.m., Adam Preble wrote:
> Given this in Python 3.6.8:
>
> from dis import dis
>
> def import_from_test():
> from sys import path
>
>>>> dis(import_from_test)
> 2 0 LOAD_CONST 1 (0)
> 2 LOAD_CONST 2 (('path',))
> 4 IMPORT_NAME 0 (sys)
> 6 IMPORT_FROM 1 (path)
> 8 STORE_FAST 0 (path)
> 10 POP_TOP
> 12 LOAD_CONST 0 (None)
> 14 RETURN_VALUE
>
> I don't understand why there's a POP_TOP there that I don't get for an import_name grammatical statement.
>
> IMPORT_NAME needs to eat the top two entries of the stack for level and the from-list. BTW I don't know what level is for either since my science projects have always had it be zero, but that's another question.
>
> IMPORT_NAME will the push the module on to the stack.
>
> IMPORT_FROM will import path from the module on the stack, and push that result on the stack.
>
> STORE_FAST will store path for use, finally "modifying the namespace."
>
> At this point, my conceptual stack is empty. If I POP_TOP then I have nothing to pop and the world would end. Yet, it doesn't. What am I missing?
You can get an idea of what you're missing if you import multiple names
from a module at once:
>>> def f():
... from sys import path, argv
...
>>> dis.dis(f)
2 0 LOAD_CONST 1 (0)
2 LOAD_CONST 2 (('path', 'argv'))
4 IMPORT_NAME 0 (sys)
6 IMPORT_FROM 1 (path)
8 STORE_FAST 0 (path)
10 IMPORT_FROM 2 (argv)
12 STORE_FAST 1 (argv)
14 POP_TOP
16 LOAD_CONST 0 (None)
18 RETURN_VALUE
As shown here (and confirmed by the doc of the IMPORT_FROM opcode),
IMPORT_FROM loads an attribute from the module on top of the stack, but
doesn't pop the module. The POP_TOP instruction is what does.
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