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[Tut] How to Assign the Result of eval() to a Python Variable? - xSicKxBot - 05-21-2022
How to Assign the Result of eval() to a Python Variable?
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<p class="has-base-background-color has-background"><img src="https://s.w.org/images/core/emoji/13.1.0/72x72/1f4ac.png" alt="?" class="wp-smiley" style="height: 1em; max-height: 1em;" /> <strong>Question</strong>: Say you have an expression you want to execute using the <code><a href="https://blog.finxter.com/python-eval/" data-type="post" data-id="19204" target="_blank" rel="noreferrer noopener">eval()</a></code> function. How to store the result of the expression in a Python variable <code>my_result</code>?</p>
<p>Before I show you the solution, let’s quickly recap the <code>eval()</code> function:</p>
<h2>Recap Python eval()</h2>
<p>Python <code>eval(s)</code> parses the string argument <code>s</code> into a Python expression, runs it, and returns the result of the expression. </p>
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<iframe loading="lazy" title="Python eval() -- How to Dynamically Evaluate a Code Expression in Python" width="780" height="439" src="https://www.youtube.com/embed/2SV60ENwXVw?feature=oembed" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>
</figure>
<p><strong>Related Tutorial</strong>: <a href="https://blog.finxter.com/python-eval/" data-type="URL" data-id="https://blog.finxter.com/python-eval/" target="_blank" rel="noreferrer noopener">Python’s <code>eval()</code> built-in function</a></p>
<p>Without further ado, let’s learn how you can store the result of the <code>eval()</code> function in a Python variable:</p>
<h2>Method 1: Simple Assignment</h2>
<p class="has-global-color-8-background-color has-background">The most straightforward way to store the result of an <code>eval()</code> expression in a Python variable is to <a href="https://blog.finxter.com/python-in-place-assignment-operators/" data-type="post" data-id="33217">as</a><a rel="noreferrer noopener" href="https://blog.finxter.com/python-in-place-assignment-operators/" data-type="post" data-id="33217" target="_blank">s</a><a href="https://blog.finxter.com/python-in-place-assignment-operators/" data-type="post" data-id="33217">ign</a> the whole return value to the variable. For example, the expression <code>my_result = eval('2+2')</code> stores the result <code>4</code> in the variable <code>my_result</code>. </p>
<p>Here’s a minimal example:</p>
<pre class="EnlighterJSRAW" data-enlighter-language="python" data-enlighter-theme="" data-enlighter-highlight="" data-enlighter-linenumbers="" data-enlighter-lineoffset="" data-enlighter-title="" data-enlighter-group="">my_result = eval('2+2')
print(my_result)
# 4
</pre>
<p>This simple approach may not always work, for example, if you have a <code>print()</code> statement in the expression. </p>
<p>Read on to learn how to fix this issue next and learn something new!</p>
<h2>Method 2: Redirect Standard Output</h2>
<p>This method assumes you have a <code><a rel="noreferrer noopener" href="https://blog.finxter.com/python-print/" data-type="post" data-id="20731" target="_blank">print()</a></code> statement within the expression passed into the <code><a rel="noreferrer noopener" href="https://blog.finxter.com/python-eval/" data-type="post" data-id="19204" target="_blank">eval()</a></code> function such as shown in the following three examples:</p>
<ul>
<li><code>eval('print(2+2)')</code></li>
<li><code>eval('print([1, 2, 3, 4] + [5, 6])')</code></li>
<li><code>eval('print(2+2*0)')</code></li>
</ul>
<p>To get the output and store it in a variable <code>my_result</code>, you need to temporarily <a href="https://blog.finxter.com/7-easy-steps-to-redirect-your-standard-output-to-a-variable-python/" data-type="post" data-id="365385" target="_blank" rel="noreferrer noopener">redirect the standard output</a> to the variable. </p>
<p>The following code shows you how to accomplish exactly this:</p>
<pre class="EnlighterJSRAW" data-enlighter-language="python" data-enlighter-theme="" data-enlighter-highlight="15-16" data-enlighter-linenumbers="" data-enlighter-lineoffset="" data-enlighter-title="" data-enlighter-group=""># 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
</pre>
<p>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.</p>
<p class="has-global-color-8-background-color has-background"><img src="https://s.w.org/images/core/emoji/13.1.0/72x72/1f440.png" alt="?" class="wp-smiley" style="height: 1em; max-height: 1em;" /> <strong>Related Article:</strong> <a href="https://blog.finxter.com/7-easy-steps-to-redirect-your-standard-output-to-a-variable-python/" data-type="post" data-id="365385">7 Easy Steps to Redirect Your Standard Output to a Variable (Python)</a></p>
<p>Note that this approach even works if you don’t have a <code>print()</code> statement in the original <code>eval()</code> expression because you can always artificially add the <code>print()</code> statement around the original expression like so:</p>
<ul>
<li><code>eval('2+2')</code> becomes <code>eval('print(2+2)')</code></li>
<li><code>eval('2+2*0')</code> becomes <code>eval('print(2+2*0)')</code></li>
<li><code>eval('[1, 2, 3] + [4, 5]')</code> becomes <code>eval('print([1, 2, 3] + [4, 5])')</code></li>
</ul>
<p>Even if it’s a bit clunky, after applying this short trick, you can redirect the standard output and store the result of <strong><em>any</em></strong> <code>eval()</code> expression in a variable.</p>
<h2>Method 3: Use exec()</h2>
<p class="has-global-color-8-background-color has-background">Using only Python’s <code>eval()</code> function, you cannot define variables inside the expression to be evaluated. However, you <em>can </em><strong>define a variable inside the <code>exec()</code> function</strong> 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()</code> expression!</p>
<p>Here’s how that works in a minimal example:</p>
<pre class="EnlighterJSRAW" data-enlighter-language="generic" data-enlighter-theme="" data-enlighter-highlight="" data-enlighter-linenumbers="" data-enlighter-lineoffset="" data-enlighter-title="" data-enlighter-group="">exec('my_result = 40 + 2')
print(my_result)
# 42</pre>
<p>Variable <code>my_result</code> is only defined in the string expression passed into <code>exec()</code>, but you can use it in the code like it was part of the original source code.</p>
<h2>Recap exec() vs eval()</h2>
<p>Python’s <code>exec()</code> function takes a Python program, as a string or executable object, and runs it. The <code>eval()</code> function evaluates an expression and returns the result of this expression. There are two main differences:</p>
<ul>
<li><code>exec()</code> can execute all Python source code, whereas <code>eval()</code> can only evaluate expressions.</li>
<li><code>exec()</code> always returns <code>None</code>, whereas <code>eval()</code> returns the result of the evaluated expression.</li>
<li><code>exec()</code> can import modules, whereas <code>eval()</code> cannot.</li>
</ul>
<p>You can learn more about the <code>exec()</code> function here:</p>
<ul>
<li><strong>Related Article:</strong> <a href="https://blog.finxter.com/python-exec/" data-type="post" data-id="18860" target="_blank" rel="noreferrer noopener">Python <code>exec()</code> — A Hacker’s Guide to A Dangerous Function</a></li>
</ul>
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<iframe loading="lazy" title="Python exec() — A Hacker’s Guide to A Dangerous Function" width="780" height="439" src="https://www.youtube.com/embed/DHpDA4QDGiU?feature=oembed" frameborder="0" allow="accelerometer; autoplay; clipboard-write; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe>
</div>
</figure>
<h2>Where to Go From Here?</h2>
<p>Enough theory. Let’s get some practice!</p>
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<p>Do you want to stop learning with toy projects and focus on practical code projects that earn you money and solve real problems for people?</p>
<p class="has-global-color-8-background-color has-background"><img src="https://s.w.org/images/core/emoji/13.1.0/72x72/1f680.png" alt="?" class="wp-smiley" style="height: 1em; max-height: 1em;" /> If your answer is <strong><em>YES!</em></strong>, consider becoming a <a rel="noreferrer noopener" href="https://blog.finxter.com/become-python-freelancer-course/" data-type="page" data-id="2072" target="_blank">Python freelance developer</a>! It’s the best way of approaching the task of improving your Python skills—even if you are a complete beginner.</p>
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