💬 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
Before I show you the solution, let’s quickly recap the
Recap Python eval()
eval(s) parses the string argument
s into a Python expression, runs it, and returns the result of the expression.
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
Here’s a minimal example:
my_result = eval('2+2') print(my_result) # 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
eval('print([1, 2, 3, 4] + [5, 6])')
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 eval(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()) # 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('[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
Here’s how that works in a minimal example:
exec('my_result = 40 + 2') print(my_result) # 42
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()
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.
eval()returns the result of the evaluated expression.
exec()can import modules, whereas
You can learn more about the
exec() function here:
- Related Article: 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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To become more successful in coding, solve more real problems for real people. That’s how you polish the skills you really need in practice. After all, what’s the use of learning theory that nobody ever needs?
You build high-value coding skills by working on practical coding projects!
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🚀 If your answer is YES!, consider becoming a Python freelance developer! It’s the best way of approaching the task of improving your Python skills—even if you are a complete beginner.
If you just want to learn about the freelancing opportunity, feel free to watch my free webinar “How to Build Your High-Income Skill Python” and learn how I grew my coding business online and how you can, too—from the comfort of your own home.
While working as a researcher in distributed systems, Dr. Christian Mayer found his love for teaching computer science students.
To help students reach higher levels of Python success, he founded the programming education website Finxter.com that has taught exponential skills to millions of coders worldwide. He’s the author of the best-selling programming books Python One-Liners (NoStarch 2020), The Art of Clean Code (NoStarch 2022), and The Book of Dash (NoStarch 2022). Chris also coauthored the Coffee Break Python series of self-published books. He’s a computer science enthusiast, freelancer, and owner of one of the top 10 largest Python blogs worldwide.
His passions are writing, reading, and coding. But his greatest passion is to serve aspiring coders through Finxter and help them to boost their skills. You can join his free email academy here.