[Tut] Python One Line Exception Handling

[Tut] Python One Line Exception Handling - Printable Version

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[Tut] Python One Line Exception Handling - xSicKxBot - 07-29-2020

Python One Line Exception Handling

<div>Summary: You can accomplish one line exception handling with the <code>exec()</code> workaround by passing the one-linerized <code>try</code>/<code>except</code> block as a string into the function like this: <code>exec('try:print(x)\nexcept:print("Exception!")')</code>. This general method works for all custom, even multi-line, try and except blocks. However, you should avoid this one-liner code due to the bad readability.
Surprisingly, there has been a <a href="https://mail.python.org/pipermail/python-ideas/2013-March/019762.html" target="_blank" rel="noreferrer noopener" title="https://mail.python.org/pipermail/python-ideas/2013-March/019762.html">discussion </a>about one-line exception handling on the official Python mailing list in 2013. However, since then, there has been no new “One-Line Exception Handling” feature in Python. So, we need to stick with the methods shown in this tutorial. But they will be fun—promised! 
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Let’s dive into the problem:
Problem: How to write the try/except block in a single line of Python code?
Example: Consider the following try/except block. 
<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="">try: print(x)
except: print('Exception!')</pre>
Solution: Before we dive into each of the three methods to solve this problem, let’s have a quick overview in our interactive code shell:
 <iframe height="700px" width="100%" src="https://repl.it/@finxter/StripedKindMemwatch?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> 
Exercise: Run the code. Why are there only three lines of output? Modify the code such that each of the four methods generate an output!
<h2>Method 1: Ternary Operator</h2>
The following method to replace a simple try/except statement is based on the <a href="https://blog.finxter.com/python-one-line-ternary/" title="Python One Line Ternary" target="_blank" rel="noreferrer noopener">ternary operator</a>.
Ternary Operator Background: The most basic ternary operator <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>.
You can use the <code>dir()</code> function to check if the variable name <code>'x'</code> already has been defined by using the condition <code>'x' in dir()</code>. If the condition evaluates to <code>True</code>, you run the try block. If it evaluates to <code>False</code>, you run the except block. 
<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
print(x) if 'x' in dir() else print('Exception!')</pre>
The output of this code snippet as a standalone code is:
<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="">Exception!</pre>
This is because the variable <code>x</code> is not defined and it doesn’t appear in the variable name directory:
<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="">print(dir())
# ['__annotations__', '__builtins__', '__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__']</pre>
For example, if you define variable x beforehand, the code would run through:
<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 = 2
print(x) if 'x' in dir() else print('Exception!')</pre>
A disadvantage of this technique is that you need to know the kinds of exceptions that may occur. Also, it becomes harder to express multi-line try and except blocks. In this case, it’s often better to use the explicit try/except statements in the first place!
<h2>Method 2: exec()</h2>
The <code>exec()</code> function takes a string and runs the string as if it was a piece of source code. This way, you can compress any algorithm in a single line. You can also compress the try/except statement into a single line of code this way!
<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
exec('try:print(x)\nexcept:print("Exception!")')</pre>
If you’d define the variable x beforehand, the result would be different:
<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('x=2\n' + 'try:print(x)\nexcept:print("Exception!")')
# 2</pre>
Now, the variable 2 is defined and the try block of the statement runs without exception. 
<h2>Method 3: Contextlib Suppress + With Statement</h2>
If you’re not really interested in the except part and you just need to catch exceptions, this method may be for you:
<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 3
from contextlib import suppress
with suppress(NameError): print(x)</pre>
You use a <a href="https://blog.finxter.com/python-one-line-with-statement/" title="Python One Line With Statement" target="_blank" rel="noreferrer noopener">with block and write it into a single line</a>. The object you pass into the with block must define two functions <code>__enter__()</code> and <code>__exit__()</code>. You use the <code><a href="https://docs.python.org/3/library/contextlib.html#contextlib.suppress" target="_blank" rel="noreferrer noopener" title="https://docs.python.org/3/library/contextlib.html#contextlib.suppress">suppress()</a></code> method from the <code>contextlib</code> package to create such an object (a so-called context manager) that suppresses the occurrence of the NameError. The beauty of the with block is that it ensures that all errors on the <code>with</code> object are handled and the object is properly closed through the <code>__exit__()</code> method. 
The disadvantage or advantage—depending on your preferences—is that there’s no except block.
Thanks for reading this blog tutorial! <img src="https://s.w.org/images/core/emoji/12.0.0-1/72x72/1f642.png" alt="?" class="wp-smiley" style="height: 1em; max-height: 1em;" />
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