Python Why `import` does not work in this case (in `exec`)? [duplicate]

Question

Sorry but the situation a bit complicated that I can't describe it clearly in the title.

So this is the script to be imported later in exec:

# script_to_be_imported.py

def f():
    print("Hello World!")

And this is my main script:

# main.py

script = """
import script_to_be_imported
def function_to_use_the_imported_script():
    script_to_be_imported.f()

function_to_use_the_imported_script()
"""


def function_invokes_exec():
    exec(script)


function_invokes_exec()

I am using Python 3.11.4 (tags/v3.11.4:d2340ef, Jun 7 2023, 05:45:37) [MSC v.1934 64 bit (AMD64)] on win32, and it tells me that:

Traceback (most recent call last):
  File "C:\Users\yueyinqiu\Documents\MyTemporaryFiles\stackoverflow\importInExec\main.py", line 16, in <module>
    function_invokes_exec()
  File "C:\Users\yueyinqiu\Documents\MyTemporaryFiles\stackoverflow\importInExec\main.py", line 13, in function_invokes_exec
    exec(script)
  File "<string>", line 6, in <module>
  File "<string>", line 4, in function_to_use_the_imported_script
NameError: name 'script_to_be_imported' is not defined

But when I make some small changes which I think they are unrelated, it could work correctly.

For example, it works when exec is invoked outside the function:

# main.py

script = """
import script_to_be_imported
def function_to_use_the_imported_script():
    script_to_be_imported.f()

function_to_use_the_imported_script()
"""

exec(script)

and also works when:

# main.py

script = """
import script_to_be_imported
script_to_be_imported.f()
"""


def function_invokes_exec():
    exec(script)


function_invokes_exec()

It even works when a value is passed to global although it's just an empty dictionary:

# main.py

script = """
import script_to_be_imported
def function_to_use_the_imported_script():
    script_to_be_imported.f()

function_to_use_the_imported_script()
"""


def function_invokes_exec():
    exec(script, {})


function_invokes_exec()

So have I misunderstood something? Or it is a bug of python?

Answer 1

From documentation:

...if the optional parts are omitted, the code is executed in the "current scope".

The current scope here is the local namespace of the function_invokes_exec function.

If you look closely, the error comes from this line script_to_be_imported.f() inside function_to_use_the_imported_script function.

before function_to_use_the_imported_script gets called, what do we have in the local scope of function_invokes_exec?

1. script_to_be_imported
2. function_to_use_the_imported_script

Now when function_to_use_the_imported_script gets called and encounters a name, it should either look at "its" local namespace or global module(forget about nonlocal namespace for now^[1]). script_to_be_imported is not in its local, is it inside the global namespace? No! So it can't find it. Where is it? It's in function_invokes_exec's local namespace. Does function_to_use_the_imported_script have access to that namespace? No.

Beside your mentioned cases, if you import script_to_be_imported in the global namespace it would also work:

# main.py
import script_to_be_imported

script = """
def function_to_use_the_imported_script():
    script_to_be_imported.f()

function_to_use_the_imported_script()
"""

def function_invokes_exec():
    exec(script)

function_invokes_exec()

---

^[1]: Actually if a function is defined "inside" another function, it has access to the enclosing namespace. It's called "nonlocal" namespace. But here since function_to_use_the_imported_script hasn't been defined there, I didn't talk about it. Thanks to @Kellybundy for bringing the point up.