When Do You Use an Ellipsis in Python?

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September 19, 2022
When Do You Use an Ellipsis in Python?
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In English writing, you can use the ellipsis to indicate that you’re leaving something out. Essentially, you use three dots (...) to replace the content. But the ellipsis doesn’t only exist in prose—you may have seen three dots in Python source code, too.

The ellipsis literal (...) evaluates to Python’s Ellipsis. Because Ellipsis is a built-in constant, you can use Ellipsis or ... without importing it:

>>> 
>>> ...
Ellipsis

>>> Ellipsis
Ellipsis

>>> ... is Ellipsis
True

Although three dots may look odd as Python syntax, there are situations where using ... can come in handy. But when should you use Ellipsis in Python?

In Short: Use the Ellipsis as a Placeholder in Python

While you can use ... and Ellipsis interchangeably, you’ll commonly opt for ... in your code. Similar to using three dots in English to omit content, you can use the ellipsis in Python as a placeholder for unwritten code:

# ellipsis_example.py

def do_nothing():
    ...

do_nothing()

When you run ellipsis_example.py and execute do_nothing(), then Python runs without complaining:

$ python ellipsis_example.py
$

There’s no error when you execute a function in Python that contains only ... in the function body. That means you can use an ellipsis as a placeholder similar to the pass keyword.

Using three dots creates minimal visual clutter. So, it can be convenient to replace irrelevant code when you’re sharing parts of your code online.

A common time when you omit code is when you work with stubs. You can think of stubs as stand-ins for real functions. Stubs can come in handy when you only need a function signature but don’t want to execute the code in the function body. For example, you’d probably want to prevent external requests when you’re developing an application.

Say you have a Flask project where you’ve created your own visitor counter in custom_stats.count_visitor(). The count_visitor() function is connected to the database where you track the number of visitors. To not count yourself when you test your application in debug mode, you can create a stub of count_visitor():

 1# app.py
 2
 3from flask import Flask
 4
 5from custom_stats import count_visitor
 6
 7app = Flask(__name__)
 8
 9if app.debug:
10    def count_visitor(): ...
11
12@app.route("/")
13def home():
14    count_visitor()
15    return "Hello, world!"

Because the content of count_visitor() doesn’t matter in this case, it’s a good idea to use the ellipsis in the function body. Python calls count_visitor() without error or unwanted side effects when you run your Flask application in debug mode:

$ flask --app app --debug run
 * Serving Flask app 'app'
 * Debug mode: on

Read the full article at https://realpython.com/python-ellipsis/ »

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September 19, 2022 at 07:30PM
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