In this post we’ll go through the different uses of the _ character in Python. Like with many things in Python, we’ll see that different usages of _ are mostly (not always!) a matter of convention. Here are the five different cases we’ll cover:

  1. Single lone underscore (e.g. _)
  2. Single underscore before a name (e.g. _total)
  3. Single underscore after a name (e.g. total_)
  4. Single underscore in numeric literals (e.g. 100_000)
  5. Double underscore before a name (e.g. __total)
  6. Double underscore before and after a name (e.g. __init__)

1. Single lone underscore (_)

This is typically used in 3 cases:

The _ name points to the result…

This post discusses Python’s NotImplemented built-in constant/type; what it is, what it means and when it should be used.

>>> type(NotImplemented)
<type 'NotImplementedType'>

NotImplemented is one of Python’s six constants living in the built-in namespace. The others are False, True, None, Ellipsis and __debug__. Similar to Ellipsis, NotImplemented can be reassigned (shadowed). Assignments to it, even as an attribute name, do not raise a SyntaxError. So it isn’t really a “real/true” constant. Of course, we should never ever re-assign it. But, for completeness:

>>> None = 'hello'
SyntaxError: can't assign to keyword
>>> NotImplemented
>>> NotImplemented = 'do not'
>>> NotImplemented
'do not'…

This post discusses Python’s from <module> import * and from <package> import *, how they behave and why it may be (is!) a bad idea to use them.

from <module> import * means “I want access to all the names in <module> that I’m meant to have access to”. So, let’s say we have the following

# something.pypublic_variable = 42
_private_variable = 141
def public_function():
print("I'm a public function! yay!")
def _private_function():
print("Ain't nobody accessing me from another module...usually")
class PublicClass(object):
class _WeirdClass(object):

In the Python Interpreter, we can execute from something import * and see…

This post discusses the else clause in Python’s try statement. Although this particular use of else may not be as forgotten and controversial as its use in Python loops, a gentle reminder of how it works and when it’s useful may still be beneficial. Please note that this post only focuses on the else clause of the try statement and does not intend to cover the entire try statement. For a pretty good initial explanation of how the entire try statement works, refer to the official Python documentation on this topic.

A “normal” stripped down try statement seen regularly looks…

In this post we’ll discuss Python’s for...else and while...else syntax, one of the most rarely used and misunderstood features in Python.

Both for and while loops in Python also take an optional else suite (like the if statement and the try statement do), which executes if the loop iteration completes normally. In other words, the else suite will be executed if we don’t exit the loop in any way other than its natural way. So, no break statements, no return statement, or no exceptions being raised inside the loop. Consider a simple (and useless) example:

In the code above…

Shahriar Tajbakhsh

Co-founder & CTO at Metaview. Previously, Palantir, Osper, and Morgan Stanley. I love a silly salmon with a good egg. @s16h_ on Twitter.

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