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

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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

Code:

eval()

function. How to store the result of the expression in a Python variable

Code:

my_result

?

Before I show you the solution, let’s quickly recap the

Code:

eval()

function:

Recap Python eval()

Python

Code:

eval(s)

parses the string argument

Code:

s

into a Python expression, runs it, and returns the result of the expression.



Related Tutorial: Python’s

Code:

eval()

built-in function

Without further ado, let’s learn how you can store the result of the

Code:

eval()

function in a Python variable:

Method 1: Simple Assignment

The most straightforward way to store the result of an

Code:

eval()

expression in a Python variable is to assign the whole return value to the variable. For example, the expression

Code:

my_result = eval('2+2')

stores the result

Code:

4

in the variable

Code:

my_result

.

Here’s a minimal example:

Code:

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

This simple approach may not always work, for example, if you have a

Code:

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

Code:

print()

statement within the expression passed into the

Code:

eval()

function such as shown in the following three examples:

• Code:

  eval('print(2+2)')
  
  
• Code:

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

  eval('print(2+2*0)')

To get the output and store it in a variable

Code:

my_result

, you need to temporarily redirect the standard output to the variable.

The following code shows you how to accomplish exactly this:

Code:

# 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

Code:

print()

statement in the original

Code:

eval()

expression because you can always artificially add the

Code:

print()

statement around the original expression like so:

• Code:

  eval('2+2')
  becomes
  Code:

  eval('print(2+2)')
  
  
• Code:

  eval('2+2*0')
  becomes
  Code:

  eval('print(2+2*0)')
  
  
• Code:

  eval('[1, 2, 3] + [4, 5]')
  becomes
  Code:

  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

Code:

eval()

expression in a variable.

Method 3: Use exec()

Using only Python’s

Code:

eval()

function, you cannot define variables inside the expression to be evaluated. However, you can define a variable inside the

Code:

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

Code:

exec()

expression!

Here’s how that works in a minimal example:

Code:

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

Variable

Code:

my_result

is only defined in the string expression passed into

Code:

exec()

, but you can use it in the code like it was part of the original source code.

Recap exec() vs eval()

Python’s

Code:

exec()

function takes a Python program, as a string or executable object, and runs it. The

Code:

eval()

function evaluates an expression and returns the result of this expression. There are two main differences:

• Code:

  exec()
  can execute all Python source code, whereas
  Code:

  eval()
  can only evaluate expressions.
  
  
• Code:

  exec()
  always returns
  Code:

  None
  , whereas
  Code:

  eval()
  returns the result of the evaluated expression.
  
  
• Code:

  exec()
  can import modules, whereas
  Code:

  eval()
  cannot.
  

You can learn more about the

Code:

exec()

function here:

• Related Article: Python
  Code:

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

Where to Go From Here?

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