List of tuples to two lists

This is hands down my most useful python one liner, and I am embarrassed to say I have frequented the same Stack Overflow entry too many times that I should have remembered it by now.

It concerns converting a list of tuples to two lists. This is very common in my data analysis methodology, as I typically append tuples for X and Y data values mainly for plotting purposes. The reason for using tuples (instead of two lists) is to allow easy sorting and filtering of values after appended to a list. However in some cases I then need to convert it back to two separate lists (matplotlib). For these times Google knows I have forgotten this and immediately gives me the post just by typing ‘python list of tup’ into the search.

Here is the Stack Overflow link, and here is the beautiful one liner.

X, Y = map(list, zip(*list_of_tuples))

For example:

lot = [(1, 2), (3, 4), (5, 6)]
X, Y = map(list, zip(*lot))
print(X) # [1, 3, 5]
print(Y) # [2, 4, 6]

So now instead of doing the naive plot with:

import matplotlib.pyplot as plt
lot = [(1, 2), (3, 4), (5, 6)]
plt.plot(*lot, 'ko') # does not do what you think it does
plt.show()

We actually want:

import matplotlib.pyplot as plt
lot = [(1, 2), (3, 4), (5, 6)]
plt.plot(*map(list, zip(*lot)), 'ko') # does do what you want
plt.show()

I hope that by writting this post it will forever ingrain it into my memory.

Written by Thomas Stainer who likes to develop software for applications mainly in maths and physics, but also to solve everyday problems. Check out my GitHub page here.