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

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💬 Question: Say you have an expression you want to execute using the eval() 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 eval() function:

Recap Python eval()

Python eval(s) parses the string argument s into a Python expression, runs it, and returns the result of the expression.

Python eval() -- How to Dynamically Evaluate a Code Expression in Python

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

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

Method 1: Simple Assignment

The most straightforward way to store the result of an eval() expression in a Python variable is to assign the whole return value to the variable. For example, the expression my_result = eval('2+2') stores the result 4 in the variable my_result.

Here’s a minimal example:

my_result = eval('2+2')
# 4

This simple approach may not always work, for example, if you have a print() statement in the expression.

Read on to learn how to fix this issue next and learn something new!

Method 2: Redirect Standard Output

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

  • eval('print(2+2)')
  • eval('print([1, 2, 3, 4] + [5, 6])')
  • eval('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 eval() expression here

# 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())
# hello world

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 eval() expression because you can always artificially add the print() statement around the original expression like so:

  • eval('2+2') becomes eval('print(2+2)')
  • eval('2+2*0') becomes eval('print(2+2*0)')
  • eval('[1, 2, 3] + [4, 5]') becomes eval('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 eval() expression in a variable.

Method 3: Use exec()

Using only Python’s eval() function, you cannot define variables inside the expression to be evaluated. However, 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 termination of the exec() expression!

Here’s how that works in a minimal example:

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

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

Recap exec() vs eval()

Python’s exec() function takes a Python program, as a string or executable object, and runs it. The eval() function evaluates an expression and returns the result of this expression. There are two main differences:

  • exec() can execute all Python source code, whereas eval() can only evaluate expressions.
  • exec() always returns None, whereas eval() returns the result of the evaluated expression.
  • exec() can import modules, whereas eval() cannot.

You can learn more about the exec() function here:

Python exec() — A Hacker’s Guide to A Dangerous Function

Where to Go From Here?

Enough theory. Let’s get some practice!

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