How to split a dataframe string column into two columns?
TL;DR version:
For the simple case of:
- I have a text column with a delimiter and I want two columns
The simplest solution is:
df[['A', 'B']] = df['AB'].str.split(' ', 1, expand=True)
You must use expand=True
if your strings have a non-uniform number of splits and you want None
to replace the missing values.
Notice how, in either case, the .tolist()
method is not necessary. Neither is zip()
.
In detail:
Andy Hayden's solution is most excellent in demonstrating the power of the str.extract()
method.
But for a simple split over a known separator (like, splitting by dashes, or splitting by whitespace), the .str.split()
method is enough1. It operates on a column (Series) of strings, and returns a column (Series) of lists:
>>> import pandas as pd>>> df = pd.DataFrame({'AB': ['A1-B1', 'A2-B2']})>>> df AB0 A1-B11 A2-B2>>> df['AB_split'] = df['AB'].str.split('-')>>> df AB AB_split0 A1-B1 [A1, B1]1 A2-B2 [A2, B2]
1: If you're unsure what the first two parameters of .str.split()
do,I recommend the docs for the plain Python version of the method.
But how do you go from:
- a column containing two-element lists
to:
- two columns, each containing the respective element of the lists?
Well, we need to take a closer look at the .str
attribute of a column.
It's a magical object that is used to collect methods that treat each element in a column as a string, and then apply the respective method in each element as efficient as possible:
>>> upper_lower_df = pd.DataFrame({"U": ["A", "B", "C"]})>>> upper_lower_df U0 A1 B2 C>>> upper_lower_df["L"] = upper_lower_df["U"].str.lower()>>> upper_lower_df U L0 A a1 B b2 C c
But it also has an "indexing" interface for getting each element of a string by its index:
>>> df['AB'].str[0]0 A1 AName: AB, dtype: object>>> df['AB'].str[1]0 11 2Name: AB, dtype: object
Of course, this indexing interface of .str
doesn't really care if each element it's indexing is actually a string, as long as it can be indexed, so:
>>> df['AB'].str.split('-', 1).str[0]0 A11 A2Name: AB, dtype: object>>> df['AB'].str.split('-', 1).str[1]0 B11 B2Name: AB, dtype: object
Then, it's a simple matter of taking advantage of the Python tuple unpacking of iterables to do
>>> df['A'], df['B'] = df['AB'].str.split('-', 1).str>>> df AB AB_split A B0 A1-B1 [A1, B1] A1 B11 A2-B2 [A2, B2] A2 B2
Of course, getting a DataFrame out of splitting a column of strings is so useful that the .str.split()
method can do it for you with the expand=True
parameter:
>>> df['AB'].str.split('-', 1, expand=True) 0 10 A1 B11 A2 B2
So, another way of accomplishing what we wanted is to do:
>>> df = df[['AB']]>>> df AB0 A1-B11 A2-B2>>> df.join(df['AB'].str.split('-', 1, expand=True).rename(columns={0:'A', 1:'B'})) AB A B0 A1-B1 A1 B11 A2-B2 A2 B2
The expand=True
version, although longer, has a distinct advantage over the tuple unpacking method. Tuple unpacking doesn't deal well with splits of different lengths:
>>> df = pd.DataFrame({'AB': ['A1-B1', 'A2-B2', 'A3-B3-C3']})>>> df AB0 A1-B11 A2-B22 A3-B3-C3>>> df['A'], df['B'], df['C'] = df['AB'].str.split('-')Traceback (most recent call last): [...] ValueError: Length of values does not match length of index>>>
But expand=True
handles it nicely by placing None
in the columns for which there aren't enough "splits":
>>> df.join(... df['AB'].str.split('-', expand=True).rename(... columns={0:'A', 1:'B', 2:'C'}... )... ) AB A B C0 A1-B1 A1 B1 None1 A2-B2 A2 B2 None2 A3-B3-C3 A3 B3 C3
There might be a better way, but this here's one approach:
row 0 00000 UNITED STATES 1 01000 ALABAMA 2 01001 Autauga County, AL 3 01003 Baldwin County, AL 4 01005 Barbour County, AL
df = pd.DataFrame(df.row.str.split(' ',1).tolist(), columns = ['fips','row'])
fips row0 00000 UNITED STATES1 01000 ALABAMA2 01001 Autauga County, AL3 01003 Baldwin County, AL4 01005 Barbour County, AL
You can extract the different parts out quite neatly using a regex pattern:
In [11]: df.row.str.extract('(?P<fips>\d{5})((?P<state>[A-Z ]*$)|(?P<county>.*?), (?P<state_code>[A-Z]{2}$))')Out[11]: fips 1 state county state_code0 00000 UNITED STATES UNITED STATES NaN NaN1 01000 ALABAMA ALABAMA NaN NaN2 01001 Autauga County, AL NaN Autauga County AL3 01003 Baldwin County, AL NaN Baldwin County AL4 01005 Barbour County, AL NaN Barbour County AL[5 rows x 5 columns]
To explain the somewhat long regex:
(?P<fips>\d{5})
- Matches the five digits (
\d
) and names them"fips"
.
The next part:
((?P<state>[A-Z ]*$)|(?P<county>.*?), (?P<state_code>[A-Z]{2}$))
Does either (|
) one of two things:
(?P<state>[A-Z ]*$)
- Matches any number (
*
) of capital letters or spaces ([A-Z ]
) and names this"state"
before the end of the string ($
),
or
(?P<county>.*?), (?P<state_code>[A-Z]{2}$))
- matches anything else (
.*
) then - a comma and a space then
- matches the two digit
state_code
before the end of the string ($
).
In the example:
Note that the first two rows hit the "state" (leaving NaN in the county and state_code columns), whilst the last three hit the county, state_code (leaving NaN in the state column).