How to Assign the Result of exec() to a Python Variable?

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💬 Question: Say you have an expression you want to execute using the exec() function. How to store the result of the expression in a Python variable my_result?

Before I show you the solution, let’s quickly recap the exec() function:

Recap Python exec()

Python’s exec() function executes the Python code you pass as a string or executable object argument.

This is called dynamic execution because, in contrast to normal static Python code, you can generate code and execute it at runtime.

This way, you can run programmatically-created Python code.

Related Tutorial: Python’s exec() built-in function

Without further ado, let’s learn how you can store the result of the exec() function in a Python variable:

Method 1: Define Variable in String Representation of Code

You can define a variable inside the exec() function that will then be added to the global namespace. Thus, you can access the defined variable in your code after the termination of the exec() expression!

Here’s how that works in a minimal example:

exec('my_result = 40 + 2')
print(my_result)
# 42

Variable my_result is only defined in the string expression passed into exec(). You can use it in the code like it was part of the original source code.

Method 2: How to Replace Value of a Variable in exec()

To replace a variable defined outside the exec() function call, you can simply do so using an f-string and pass the new value as the right-hand side of the assignment operator within the string representation of the code to be executed.

Here’s a minimal example where we overwrite the old_variable with the new string 'hello finxter' by using the expression:

exec(f'old_variable = "{new_string}"')

old_variable = 'hello world'
new_string = 'hello finxter'

# Replace old_old variable with new string 'hello finxter'
exec(f'old_variable = "{new_string}"')

print(old_variable)
# hello finxter

Method 3: How to Replace Global Variable in exec()?

If you use the previous approach to replace a global variable defined outside the exec() expression that is called in a local context, the assignment operator will work on a local alias variable.

The global variable to be replaced will remain the same!

This can be seen in the following code example, where the global old_variable is not replaced with the new string 'hello finxter' because of the local scope of the function f that creates its own namespace with its own old_variable that overshadows the global old_variable.

old_variable = 'hello world'
new_string = 'hello finxter'

def f():
    # Replace old_old variable with new string 'hello finxter'
    exec(f'old_variable = "{new_string}"')

f()

print(old_variable)
# hello world

To overwrite a global variable within the exec() function, use the global keyword, followed by an empty space, the name of the global variable, a semicolon ; to close the statement, and the expression to overwrite the global variable like so: exec(f'global old_variable; old_variable = "{new_string}"')

Here’s the full example:

old_variable = 'hello world'
new_string = 'hello finxter'

def f():
    # Replace old_old variable with new string 'hello finxter'
    exec(f'global old_variable; old_variable = "{new_string}"')


f()

print(old_variable)
# hello finxter

Related tutorial:

Method 4: Redirect Standard Output

Okay, let’s do something crazy, shall we? 🤯

Let’s redirect the standard output and print directly to the variable!

This method assumes you have a print() statement within the expression passed into the exec() function such as shown in the following three examples:

  • exec('print(2+2)')
  • exec('print([1, 2, 3, 4] + [5, 6])')
  • exec('print(2+2*0)')

To get the output and store it in a variable my_result, you need to temporarily redirect the standard output to the variable.

The following code shows you how to accomplish exactly this:

# Step 1: Import libraries StringIO and sys
from io import StringIO
import sys

# Step 2: Keep stdout in temporary variable
tmp = sys.stdout

# Step 3: Capture standard output using a StringIO object
my_result = StringIO()

# Step 4: Assign Standard Output Stream to StringIO object
sys.stdout = my_result

# Step 5: Print to the standard output
expression = 'print(2+2)' # any exec() expression here
exec(expression)

# Step 6: Clean up by redirecting stdout to Python shell
sys.stdout = tmp

# Step 7: Get and print the string from stdout
print('VARIABLE:', my_result.getvalue())
# VARIABLE: 4

If you need some assistance understanding this whole idea of redirecting the standard output, have a look at our in-depth guide on the Finxter blog.

👀 Related Article: 7 Easy Steps to Redirect Your Standard Output to a Variable (Python)

Note that this approach even works if you don’t have a print() statement in the original exec() expression because you can always artificially add the print() statement around the original expression like so:

  • exec('2+2') becomes exec('print(2+2)')
  • exec('2+2*0') becomes exec('print(2+2*0)')
  • exec('[1, 2, 3] + [4, 5]') becomes exec('print([1, 2, 3] + [4, 5])')

Even if it’s a bit clunky, after applying this short trick, you can redirect the standard output and store the result of any exec() expression in a variable.

👀 Related Article: How to Assign the Result of eval() to a Python Variable?

Where to Go From Here?

Enough theory. Let’s get some practice!

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