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[Tut] Python One Line If Not None - xSicKxBot - 09-11-2020
Python One Line If Not None
<div><p class="has-pale-cyan-blue-background-color has-background">To assign the result of a function <code>get_value()</code> to variable <code>x</code> if it is different from <code>None</code>, use the Walrus operator <code>if tmp := get_value(): x = tmp</code> within a single-line if block. The Walrus operator assigns the function’s return value to the variable <code>tmp</code> and returns it at the same time, so that you can check and assign it to variable <code>x</code> subsequently. </p>
<p><strong>Problem</strong>: How to assign a value to a variable if it is not equal to <code>None</code>—using only a single line of Python code? </p>
<p><strong>Example</strong>: Say, you want to assign the return value of a function get_value(), but only if it doesn’t return None. Otherwise, you want to leave the value as it is. Here’s a code 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="">import random def get_value(): if random.random()>0.5: return None return 1 # Naive approach:
x = 42
tmp = get_value()
if tmp != None: x = tmp
print(tmp)</pre>
<p>While this works, you need to execute the function <code>get_value()</code> twice which is not optimal. An alternative would be to assign the result of the <code>get_value()</code> function to a temporary variable to avoid repeated function execution:</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="">x = 42
temp = get_value()
if temp != None: x = temp
print(x)</pre>
<p>However, this seems clunky and ineffective. Is there a better way?</p>
<p>Let’s have an overview of the one-liners that conditionally assign a value to a given variable:</p>
<p> <iframe height="700px" width="100%" src="https://repl.it/@finxter/CrookedLivelyLocks?lite=true" scrolling="no" frameborder="no" allowtransparency="true" allowfullscreen="true" sandbox="allow-forms allow-pointer-lock allow-popups allow-same-origin allow-scripts allow-modals"></iframe> </p>
<p><em><strong>Exercise</strong>: Run the code. Does it always generate the same result? </em></p>
<h2>Method 1: Ternary Operator + Semicolon</h2>
<p>The most basic <a href="https://blog.finxter.com/python-one-line-ternary/" target="_blank" rel="noreferrer noopener" title="Python One Line Ternary">ternary operator</a> <code>x if c else y</code> consists of three operands <code>x</code>, <code>c</code>, and <code>y</code>. It is an expression with a return value. The ternary operator returns <code>x</code> if the Boolean expression <code>c</code> evaluates to <code>True</code>. Otherwise, if the expression <code>c</code> evaluates to <code>False</code>, the ternary operator returns the alternative <code>y</code>.</p>
<p>You can use the ternary operator to solve this problem in combination with the <a href="https://blog.finxter.com/how-to-execute-multiple-lines-in-a-single-line-python-from-command-line/" target="_blank" rel="noreferrer noopener" title="How to Execute Multiple Lines in a Single Line Python From Command-Line?">semicolon to write multiple lines of code </a>as a Python one-liner. </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=""># Method 1
tmp = get_value(); x = tmp if tmp else x</pre>
<p>You cannot run the <code>get_value()</code> function twice—to check whether it returns <code>True</code> and to assign the return value to the variable <code>x</code>. Why? Because it’s nondeterministic and may return different values for different executions.</p>
<p>Therefore, the following code would be a blunt mistake:</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="">x = get_value() if get_value() else x</pre>
<p>The variable <code>x</code> may still be <code>None</code>—even after the ternary operator has seemingly checked the condition. </p>
<figure class="wp-block-embed-youtube wp-block-embed is-type-video is-provider-youtube wp-embed-aspect-16-9 wp-has-aspect-ratio">
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<div class="ast-oembed-container"><iframe title="The Python Ternary Operator -- And a Surprising One-Liner Hack" width="1400" height="788" src="https://www.youtube.com/embed/9XXcUHXrqZ4?feature=oembed" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></div>
</div>
</figure>
<p><strong>Related articles:</strong></p>
<ul>
<li><a href="https://blog.finxter.com/python-one-line-ternary/" target="_blank" rel="noreferrer noopener" title="Python One Line Ternary">Python Ternary</a></li>
<li><a href="https://blog.finxter.com/if-then-else-in-one-line-python/" target="_blank" rel="noreferrer noopener" title="If-Then-Else in One Line Python [Video + Interactive Code Shell]">Python Single-Line If Statement</a></li>
<li><a href="https://blog.finxter.com/python-semicolons-how-they-work-and-why-haters-tell-you-to-avoid-them/" target="_blank" rel="noreferrer noopener" title="Python Semicolons: How They Work and Why Haters Tell You to Avoid Them">Python Semicolon</a></li>
</ul>
<h2>Method 2: Walrus + One-Line-If</h2>
<p>A beautiful extension of <a href="https://blog.finxter.com/how-to-check-your-python-version/" title="How to Check Your Python Version? A Helpful Guide" target="_blank" rel="noreferrer noopener">Python 3.8</a> is the <a href="https://blog.finxter.com/python-3-8-walrus-operator-assignment-expression/" title="Python 3.8 Walrus Operator (Assignment Expression)" target="_blank" rel="noreferrer noopener">Walrus operator</a>. <strong><em>The Walrus operator <code>:=</code> is an assignment operator with return value.</em></strong> Thus, it allows you to check a condition and assign a value at the same time:</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="">
# Method 2
if tmp := get_value(): x = tmp</pre>
<p>This is a very clean, readable, and Pythonic way. Also, you don’t have the redundant identity assignment in case the if condition is not fulfilled.</p>
<figure class="wp-block-embed-youtube wp-block-embed is-type-video is-provider-youtube wp-embed-aspect-16-9 wp-has-aspect-ratio">
<div class="wp-block-embed__wrapper">
<div class="ast-oembed-container"><iframe title="Python 3.8 Walrus Operator (Assignment Expression)" width="1400" height="788" src="https://www.youtube.com/embed/RGTjHAzIv7w?feature=oembed" frameborder="0" allow="accelerometer; autoplay; encrypted-media; gyroscope; picture-in-picture" allowfullscreen></iframe></div>
</div>
</figure>
<p> <strong>Related Article:</strong> <a href="https://blog.finxter.com/python-3-8-walrus-operator-assignment-expression/" target="_blank" rel="noreferrer noopener" title="Python 3.8 Walrus Operator (Assignment Expression)">The Walrus Operator in Python 3.8</a></p>
<h2>Python One-Liners Book</h2>
<p><strong>Python programmers will improve their computer science skills with these useful one-liners.</strong></p>
<figure class="wp-block-image size-medium is-resized"><a href="https://www.amazon.com/gp/product/B07ZY7XMX8" target="_blank" rel="noopener noreferrer"><img loading="lazy" src="https://blog.finxter.com/wp-content/uploads/2020/06/3D_cover-1024x944.jpg" alt="Python One-Liners" class="wp-image-10007" width="512" height="472" srcset="https://blog.finxter.com/wp-content/uploads/2020/06/3D_cover-scaled.jpg 1024w, https://blog.finxter.com/wp-content/uploads/2020/06/3D_cover-300x277.jpg 300w, https://blog.finxter.com/wp-content/uploads/2020/06/3D_cover-768x708.jpg 768w" sizes="(max-width: 512px) 100vw, 512px" /></a></figure>
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</div>
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