Wandering Thoughts archives


What the co_names attribute on Python code objects is

As a trap for the unwary, Python code objects have both a co_names and a co_varnames attribute. Since I just confused myself about which was what the other day, here is what the co_names one is.

Put simply, co_names is a tuple of names of globals and attributes that are used by the function's code. For example, if you have 'a = self.bar()' in the function, the 'bar' will show up in co_names, as will the 'foo' from 'a = foo()'.

(Perhaps I should call these 'identifiers' instead of 'names'. In Ruby and Lisp and probably elsewhere these are called symbols.)

Ultimately this is part of how the CPython bytecode interpreter is implemented. When the bytecode interpreter refers to anything but a local variable, it has to do an attribute lookup with the name to get the actual object involved. Rather than put the name that's being looked up directly in the bytecode instructions, CPython puts all the names into a table and has the instructions refer to table slots, so the LOAD_GLOBAL instruction says 'look up name 3' instead of 'look up "somevar"'. And co_names is that table (or at least a representation of that table).

Each name only appears once in co_names, no matter how many times it's used in your function and no matter if it's used in different contexts; if you have both 'obj.foo' and 'foo()' in your code, there will only be one "foo" in co_names, even though one use of the name is for an object attribute and one is for a global. As far as I can tell from reading CPython source, names are always interned strings and so are globally unique.

(The co_names table slot numbers are of course a per-function thing; slot 0 in different functions will refer to completely different names.)

python/WhatCoNamesIs written at 22:48:26; Add Comment

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