python exec command works in console but not in program

Question

The problem is that running exec() on a variable with the name of a named argument does not work in normal operation of the program, but setting a break point and executing the line of code in question at the same location seems to work.

Reproduction of the issue follows:

"""

    LEGACY CODE, CANNOT CHANGE
    vvvvvvvvvvv

"""

from collections import namedtuple


class A_Object(namedtuple('A_object', 'parameter_a' ' parameter_b')):

    def __new__(cls, *args, **kwargs):
        init = super(A_Object, cls).__new__(cls, parameter_a=[], parameter_b=[])
        return init

    """
    
        ^^^^^^^^^^^
        LEGACY CODE, CANNOT CHANGE
    
    """

    def _remove_all_data(self):
        for field in self._fields:
            print("removing field:", field)

            # This does not work in the normal execution of the program,
            # but if you set a break point and execute the same command in the python console ( in Pycharm ) it works.
            exec('self = self._replace({0}=[])'.format(field))

        return self


def driver():
    a = A_Object()

    a.parameter_a.append('a')
    a.parameter_a.append('b')
    a.parameter_a.append('c')

    a.parameter_b.append(1)
    a.parameter_b.append(2)
    a.parameter_b.append(3)

    # Try to remove all data from properties.
    a = a._remove_all_data()

    tmp = a._asdict()

    for k in tmp.keys():
        for i in range(0, len(tmp[k])):
            print('{}[{}]='.format(k,i), tmp[k][i])


if __name__ == '__main__':
    driver()

This minimum reproducible example will show that running the program will output the contents of parameter_a, and parameter_b ( That is the issue ). The desired outcome would be parameter_a, and parameter_b to be empty list. If you put a break point on the exec('self = self._replace({0}=[])'.format(field)) line and during debug open up the python console in pycharm and run that line of code "exec('self = self._replace({0}=[])'.format(field))" in the console. it will clear the parameter_a, and paramter_b ( giving the desired output ).

you cannot modify local variables with exec/eval. You shouldn't be using exec here in the first place — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 31, 2020 at 21:20
But clearly that is not exactly the case if I can use exec to modify local variables in the python console. And I agree NOT using exec/eval in this case is best practice. — Theisen
– Theisen, Commented Dec 31, 2020 at 21:24
No, you cannot, in Python 3. A debugger console is different from a normal environment (it's actually using exec already...) — juanpa.arrivillaga
– juanpa.arrivillaga, Commented Dec 31, 2020 at 21:24

kaya3 · Accepted Answer · 2020-12-31 21:06:13Z

2

You don't need exec for this: if you want a dynamically-named keyword argument, use the **kwargs syntax. You might as well also replace all the fields at once instead of calling _replace multiple times in a loop:

def _remove_all_data(self):
    return self._replace(**{ f: [] for f in self._fields })

answered Dec 31, 2020 at 21:06

kaya3

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3 Comments

Theisen Over a year ago

I forgot the ** when I first tired kwargs pass in. This solved for the desire output/outcome. Thank you. Before I mark this as the answer do you know why exec has that odd behavior of not working in this program?

juanpa.arrivillaga Over a year ago

@Theisen did you read the documentation for exec? it warns you that modifications to local variables will not work.

kaya3 Over a year ago

It's not odd behaviour - it's the expected behaviour. exec doesn't change the values of local variables like self. As the docs for the exec function advise: "Pass an explicit locals dictionary if you need to see effects of the code on locals after function exec() returns."

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