A few Python ‘Gotchas’: sorted() and list.extend()

I recently got bitten by a couple bugs in a project I was doing in Python and thought I’d share. Short and sweet, I’ll get directly to the point.

The sorted() method in Python does not sort an iterable in place. It returns the sorted iterable. Remember to catch the sorted iterable into a new variable.

my_list = [5, 3, 1, 4]
sorted(my_list, reverse = True) # Useless, unless you are printing it directly
my_list = sorted(my_list, reverse = True) # Correct

The next one is a bit more subtle. The extend() method to a list is useful if you want to add the contents of a list _individually_ to another list. But if you are adding a string to a list, the extend() method will split the string, treating it as a list of characters. The following example will make it clear.

my_str = 'randomness'
my_list = []
my_list.extend(my_str) # my_list = ['r', 'a', 'n', 'd', 'o', 'm', 'n', 'e', 's', 's']
my_list.extend(['hello', 'world']) # my_list = ['r', 'a', 'n', 'd', 'o', 'm', 'n', 'e', 's', 's', 'hello', 'world']

Small things. Must remember. So long, and happy Pythoning!

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