I need to write a custom exec function in python (for several purposes but this is not the problem here, so this custom exec which is called myExec will do exactly as exec for now).
I went into this problem :
def myExec(code):
exec(code)
code = """
a = 1
print(a)
u = [a for x in range(3)]
print(u)
"""
myExec(code)
Running this program gives
1
Traceback (most recent call last):
File "___.py", line 12, in <module>
myExec(code)
File "___.py", line 2, in myExec
exec(code, globals(), locals())
File "<string>", line 4, in <module>
File "<string>", line 4, in <listcomp>
NameError: name 'a' is not defined
So print(a) went without any problems. But the error occurs with the line u = [a for x in range(3)]. When the generator object is converted into a list, the name a seems undefined.
Note that if the line were u = [a, a, a], then, no error is raised. Nor if we use exec instead of myExec.
Any reason why and how to solve this ?
9
We can explain this behavior if we take a look at the decompiled code.
from dis import dis
def myExec(code):
dis(code)
a = 1is compiled toSTORE_NAME, so it storesaas a local variable hereprint(a)usesLOAD_NAMEto load the locala. It is a local variable, soLOAD_NAMEfinds it.- the list comprehension is compiled into a function that uses
LOAD_GLOBALfora
That’s where the error is coming from. a was created as a local variable and is accessed as a global one in the list comprehension. This results in a name error.
This also explains why the exec works in the global scope (either by calling exec in the global scope or by passing globals()). Because then STORE_NAME stores a in the current scope (which is global) and LOAD_GLOBAL can find a.
If you switch to Python3.12 which implements PEP 709 – Inlined comprehensions you will see that no extra function is created for the list comprehension and a is looked up with LOAD_NAME and can be found.
So to fix your issue: either upgrade to Python3.12 or pass the globals()
1