How do you split a list by condition in python?

Inspired by @gnibbler's great [but terse!] answer, we can apply that approach to map to multiple partitions:

from collections import defaultdict

def splitter[l, mapper]:
    """Split an iterable into multiple partitions generated by a callable mapper."""

    results = defaultdict[list]

    for x in l:
        results[mapper[x]] += [x]

    return results

Then splitter can then be used as follows:

>>> l = [1, 2, 3, 4, 2, 3, 4, 5, 6, 4, 3, 2, 3]
>>> split = splitter[l, lambda x: x % 2 == 0]  # partition l into odds and evens
>>> split.items[]
>>> [[False, [1, 3, 3, 5, 3, 3]], [True, [2, 4, 2, 4, 6, 4, 2]]]

This works for more than two partitions with a more complicated mapping [and on iterators, too]:

>>> import math
>>> l = xrange[1, 23]
>>> split = splitter[l, lambda x: int[math.log10[x] * 5]]
>>> split.items[]
[[0, [1]],
 [1, [2]],
 [2, [3]],
 [3, [4, 5, 6]],
 [4, [7, 8, 9]],
 [5, [10, 11, 12, 13, 14, 15]],
 [6, [16, 17, 18, 19, 20, 21, 22]]]

Or using a dictionary to map:

>>> map = {'A': 1, 'X': 2, 'B': 3, 'Y': 1, 'C': 2, 'Z': 3}
>>> l = ['A', 'B', 'C', 'C', 'X', 'Y', 'Z', 'A', 'Z']
>>> split = splitter[l, map.get]
>>> split.items[]
[1, ['A', 'Y', 'A']], [2, ['C', 'C', 'X']], [3, ['B', 'Z', 'Z']]]

In this short tutorial we will learn how to divide a Python list objects to multiple sublists.

Example list object

All examples in this tutorial will be based on the following programming languages list :

all_lang_lst = ['R', 'Julia', 'Python', 'Haskell', 'Java', 'Javascript']

Divide a list based with delimiter

In this simple case, the after being split, the elements will be written to a string delimited by commas followed by a space:

', '.join [all_lang_lst]

The output will be:

'R, Julia, Python, Haskell, Java, Javascript'

Split a list by condition

We might want to split a list into multiple sub lists based on a simple condition.

data_lang = ['R', 'Julia', 'Python']
other_lang = ['Java', 'Javascript', 'Haskell']
data_lst = []
other_lst = []

# divide the list values using comprehensions
data_lst = [x for x in all_lang_lst if x in data_lang]
other_lst = [x for x in all_lang_lst if x in other_lang]

#print the lists
print ["The data science related languages are:" + str[data_lst]]
print ["The other languages are: " + str[other_lst]]

Here’s the output:

The data science related languages are:['R', 'Julia', 'Python']
The other languages are: ['Haskell', 'Java', 'Javascript']

Note: as shown above we can use the join[] function to convert any list to a string.

Split Python lists by substring

In a similar fashion we can divide the list based on whether its element contains a specific sub string. We’ll start by defining a function:

def splt_by_substrng [my_lst, my_str]:
    subs_lst = []
    subs_lst_other =[]
    for e in my_lst:
        if my_str in e:
            subs_lst.append[e]
        else:
            subs_lst_other.append[e]
    return subs_lst, subs_lst_other

We’ll now call the function that we have just defined. In our case we would like to split programming languages containing the substring ‘ja’ into a separated list. The function will return two lists.

print["Here are the split lists: " + str[splt_by_substrng[all_lang_lst , "Ja"]]]

The result will be:

Here are the split lists: [['Java', 'Javascript'], ['R', 'Julia', 'Python', 'Haskell']]

Find the difference between two lists

Another way to split lists is simply by subtracting. We can convert our lists to Python sets and simply subtract them:

other_lst= set[all_lang_lst]- set[data_lang]

good = [x for x in mylist if x in goodvals]
bad  = [x for x in mylist if x not in goodvals]

[x for x in mylist if x in goodvals]

[x for x in mylist if x not in goodvals]

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