I'm out of hibernation, time I posted something.
As much as I use Python these days, there's a few things I find myself looking up regularly. At one point I just made a small crib sheet and stuck it to my monitor. I have examples on it for list comprehensions, filter, and map.
List Comprehensions
These are useful for creating modified lists from existing data without a lot of fuss. They aren't all that hard to remember, but the syntax was a bit alien to me for awhile. They're basically an expression followed by a for clause.
The below example takes an existing list, my_list and builds a new list with only the elements that are greater than 2. In this case the result is assigned right back to my_list.
my_list = [x for x in my_list if x > 2]
FilterUsing
filter is a powerful way to remove undesired elements from a list. You pass a function as the first argument, which generally returns True/False based on some criteria. The second argument is the sequence to be filtered (or any iterable object). Only the elements that return True when passed to that function will remain in the newly returned list.
Filtering is often done with a
lambda as the first function argument. A lambda is a one-off function that's defined and used in the same place. Since it's only used once, it doesn't need a name. It's so common to see
filter and
lambda together, the fact they were seperate didn't occur to me when I was learning the language.
In this example, we have a list of filenames, my_files, and we want to remove any that aren't Python scripts, ending in '.py'.
my_files = filter(lambda f: f.endswith('.py'), my_files)That is the shorter equivalent of:
def is_py_filename(filename):
return filename.endswith('.py')
my_files = filter(is_py_filename, my_files)
With
filter, passing None as the first argument instead of a function automatically removes any elements that don't evaluate to True. That includes integers or floats that are zero, as well as occurrences of
False or
None.
my_files = filter(None, my_files)
MapMapped functions let you apply a function to every element in a sequence.
def add_ten(x):
return x+10
result = map(add_ten, [1,2,3,4,5])
The value of
result would be
[11, 12, 13, 14, 15]. Of course you could also use a lambda here, too:
result = map(lambda x: x+10, [1,2,3,4,5])
So what's on your cheat sheet?
