[Python-Dev] doc behavior in class definitions (original) (raw)

Jason Orendorff jason.orendorff at gmail.com
Sat Oct 8 00:51:21 CEST 2005

Martin,

These two cases generate different bytecode.

def foo():     # foo.func_code.co_flags == 0x43
    print x    # LOAD_FAST 0
    x = 3

class Foo:     # <code object>.co_flags == 0x40
    print x    # LOAD_NAME 'x'
    x = 3

In functions, local variables are just numbered slots. (co_flags bits 1 and 2 indicate this.) The LOAD_FAST opcode is used. If the slot is empty, LOAD_FAST throws.

In other code, the local variables are actually stored in a dictionary. LOAD_NAME is used. This does a locals dictionary lookup; failing that, it falls back on the globals dictionary; and failing that, it falls back on builtins.

Why the discrepancy? Beats me. I would definitely implement what CPython does up to this point, if that's your question.

Btw, functions that use 'exec' are in their own category way out there:

def foo2():     # foo2.func_code.co_flags == 0x42
    print x     # LOAD_NAME 'x'
    exec "x=3"  # don't ever do this, it screws everything up
    print x

Pretty weird. Jython seems to implement this.

-j

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