Working with Text Data

Series and Index are equipped with a set of string processing methods that make it easy to operate on each element of the array. Perhaps most importantly, these methods exclude missing/NA values automatically. These are accessed via the str attribute and generally have names matching the equivalent (scalar) built-in string methods:

In [1]: s = pd.Series(['A', 'B', 'C', 'Aaba', 'Baca', np.nan, 'CABA', 'dog', 'cat'])

In [2]: s.str.lower()
Out[2]: 
0       a
1       b
2       c
3    aaba
4    baca
5     NaN
6    caba
7     dog
8     cat
dtype: object

In [3]: s.str.upper()
Out[3]: 
0       A
1       B
2       C
3    AABA
4    BACA
5     NaN
6    CABA
7     DOG
8     CAT
dtype: object

In [4]: s.str.len()
Out[4]: 
0    1.0
1    1.0
2    1.0
3    4.0
4    4.0
5    NaN
6    4.0
7    3.0
8    3.0
dtype: float64
In [5]: idx = pd.Index([' jack', 'jill ', ' jesse ', 'frank'])

In [6]: idx.str.strip()
Out[6]: Index(['jack', 'jill', 'jesse', 'frank'], dtype='object')

In [7]: idx.str.lstrip()
Out[7]: Index(['jack', 'jill ', 'jesse ', 'frank'], dtype='object')

In [8]: idx.str.rstrip()
Out[8]: Index([' jack', 'jill', ' jesse', 'frank'], dtype='object')

The string methods on Index are especially useful for cleaning up or transforming DataFrame columns. For instance, you may have columns with leading or trailing whitespace:

In [9]: df = pd.DataFrame(np.random.randn(3, 2),
   ...:                   columns=[' Column A ', ' Column B '], index=range(3))
   ...: 

In [10]: df
Out[10]: 
    Column A    Column B 
0    0.469112   -0.282863
1   -1.509059   -1.135632
2    1.212112   -0.173215

Since df.columns is an Index object, we can use the .str accessor

In [11]: df.columns.str.strip()
Out[11]: Index(['Column A', 'Column B'], dtype='object')

In [12]: df.columns.str.lower()
Out[12]: Index([' column a ', ' column b '], dtype='object')

These string methods can then be used to clean up the columns as needed. Here we are removing leading and trailing whitespaces, lower casing all names, and replacing any remaining whitespaces with underscores:

In [13]: df.columns = df.columns.str.strip().str.lower().str.replace(' ', '_')

In [14]: df
Out[14]: 
   column_a  column_b
0  0.469112 -0.282863
1 -1.509059 -1.135632
2  1.212112 -0.173215

Note

If you have a Series where lots of elements are repeated (i.e. the number of unique elements in the Series is a lot smaller than the length of the Series), it can be faster to convert the original Series to one of type category and then use .str.<method> or .dt.<property> on that. The performance difference comes from the fact that, for Series of type category, the string operations are done on the .categories and not on each element of the Series.

Please note that a Series of type category with string .categories has some limitations in comparison to Series of type string (e.g. you can’t add strings to each other: s + " " + s won’t work if s is a Series of type category). Also, .str methods which operate on elements of type list are not available on such a Series.

Splitting and Replacing Strings

Methods like split return a Series of lists:

In [15]: s2 = pd.Series(['a_b_c', 'c_d_e', np.nan, 'f_g_h'])

In [16]: s2.str.split('_')
Out[16]: 
0    [a, b, c]
1    [c, d, e]
2          NaN
3    [f, g, h]
dtype: object

Elements in the split lists can be accessed using get or [] notation:

In [17]: s2.str.split('_').str.get(1)
Out[17]: 
0      b
1      d
2    NaN
3      g
dtype: object

In [18]: s2.str.split('_').str[1]
Out[18]: 
0      b
1      d
2    NaN
3      g
dtype: object

It is easy to expand this to return a DataFrame using expand.

In [19]: s2.str.split('_', expand=True)
Out[19]: 
     0    1    2
0    a    b    c
1    c    d    e
2  NaN  NaN  NaN
3    f    g    h

It is also possible to limit the number of splits:

In [20]: s2.str.split('_', expand=True, n=1)
Out[20]: 
     0    1
0    a  b_c
1    c  d_e
2  NaN  NaN
3    f  g_h

rsplit is similar to split except it works in the reverse direction, i.e., from the end of the string to the beginning of the string:

In [21]: s2.str.rsplit('_', expand=True, n=1)
Out[21]: 
     0    1
0  a_b    c
1  c_d    e
2  NaN  NaN
3  f_g    h

replace by default replaces regular expressions:

In [22]: s3 = pd.Series(['A', 'B', 'C', 'Aaba', 'Baca',
   ....:                '', np.nan, 'CABA', 'dog', 'cat'])
   ....: 

In [23]: s3
Out[23]: 
0       A
1       B
2       C
3    Aaba
4    Baca
5        
6     NaN
7    CABA
8     dog
9     cat
dtype: object

In [24]: s3.str.replace('^.a|dog', 'XX-XX ', case=False)
Out[24]: 
0           A
1           B
2           C
3    XX-XX ba
4    XX-XX ca
5            
6         NaN
7    XX-XX BA
8      XX-XX 
9     XX-XX t
dtype: object

Some caution must be taken to keep regular expressions in mind! For example, the following code will cause trouble because of the regular expression meaning of $:

# Consider the following badly formatted financial data
In [25]: dollars = pd.Series(['12', '-$10', '$10,000'])

# This does what you'd naively expect:
In [26]: dollars.str.replace('$', '')
Out[26]: 
0        12
1       -10
2    10,000
dtype: object

# But this doesn't:
In [27]: dollars.str.replace('-$', '-')
Out[27]: 
0         12
1       -$10
2    $10,000
dtype: object

# We need to escape the special character (for >1 len patterns)
In [28]: dollars.str.replace(r'-\$', '-')
Out[28]: 
0         12
1        -10
2    $10,000
dtype: object

New in version 0.23.0.

If you do want literal replacement of a string (equivalent to str.replace()), you can set the optional regex parameter to False, rather than escaping each character. In this case both pat and repl must be strings:

# These lines are equivalent
In [29]: dollars.str.replace(r'-\$', '-')
Out[29]: 
0         12
1        -10
2    $10,000
dtype: object

In [30]: dollars.str.replace('-$', '-', regex=False)
Out[30]: 
0         12
1        -10
2    $10,000
dtype: object

New in version 0.20.0.

The replace method can also take a callable as replacement. It is called on every pat using re.sub(). The callable should expect one positional argument (a regex object) and return a string.

# Reverse every lowercase alphabetic word
In [31]: pat = r'[a-z]+'

In [32]: def repl(m):
   ....:     return m.group(0)[::-1]
   ....: 

In [33]: pd.Series(['foo 123', 'bar baz', np.nan]).str.replace(pat, repl)
Out[33]: 
0    oof 123
1    rab zab
2        NaN
dtype: object

# Using regex groups
In [34]: pat = r"(?P<one>\w+) (?P<two>\w+) (?P<three>\w+)"

In [35]: def repl(m):
   ....:     return m.group('two').swapcase()
   ....: 

In [36]: pd.Series(['Foo Bar Baz', np.nan]).str.replace(pat, repl)
Out[36]: 
0    bAR
1    NaN
dtype: object

New in version 0.20.0.

The replace method also accepts a compiled regular expression object from re.compile() as a pattern. All flags should be included in the compiled regular expression object.

In [37]: import re

In [38]: regex_pat = re.compile(r'^.a|dog', flags=re.IGNORECASE)

In [39]: s3.str.replace(regex_pat, 'XX-XX ')
Out[39]: 
0           A
1           B
2           C
3    XX-XX ba
4    XX-XX ca
5            
6         NaN
7    XX-XX BA
8      XX-XX 
9     XX-XX t
dtype: object

Including a flags argument when calling replace with a compiled regular expression object will raise a ValueError.

In [40]: s3.str.replace(regex_pat, 'XX-XX ', flags=re.IGNORECASE)
---------------------------------------------------------------------------
ValueError: case and flags cannot be set when pat is a compiled regex

Concatenation

There are several ways to concatenate a Series or Index, either with itself or others, all based on cat(), resp. Index.str.cat.

Concatenating a single Series into a string

The content of a Series (or Index) can be concatenated:

In [41]: s = pd.Series(['a', 'b', 'c', 'd'])

In [42]: s.str.cat(sep=',')
Out[42]: 'a,b,c,d'

If not specified, the keyword sep for the separator defaults to the empty string, sep='':

In [43]: s.str.cat()
Out[43]: 'abcd'

By default, missing values are ignored. Using na_rep, they can be given a representation:

In [44]: t = pd.Series(['a', 'b', np.nan, 'd'])

In [45]: t.str.cat(sep=',')
Out[45]: 'a,b,d'

In [46]: t.str.cat(sep=',', na_rep='-')
Out[46]: 'a,b,-,d'

Concatenating a Series and something list-like into a Series

The first argument to cat() can be a list-like object, provided that it matches the length of the calling Series (or Index).

In [47]: s.str.cat(['A', 'B', 'C', 'D'])
Out[47]: 
0    aA
1    bB
2    cC
3    dD
dtype: object

Missing values on either side will result in missing values in the result as well, unless na_rep is specified:

In [48]: s.str.cat(t)
Out[48]: 
0     aa
1     bb
2    NaN
3     dd
dtype: object

In [49]: s.str.cat(t, na_rep='-')
Out[49]: 
0    aa
1    bb
2    c-
3    dd
dtype: object

Concatenating a Series and something array-like into a Series

New in version 0.23.0.

The parameter others can also be two-dimensional. In this case, the number or rows must match the lengths of the calling Series (or Index).

In [50]: d = pd.concat([t, s], axis=1)

In [51]: s
Out[51]: 
0    a
1    b
2    c
3    d
dtype: object

In [52]: d
Out[52]: 
     0  1
0    a  a
1    b  b
2  NaN  c
3    d  d

In [53]: s.str.cat(d, na_rep='-')
Out[53]: 
0    aaa
1    bbb
2    c-c
3    ddd
dtype: object

Concatenating a Series and an indexed object into a Series, with alignment

New in version 0.23.0.

For concatenation with a Series or DataFrame, it is possible to align the indexes before concatenation by setting the join-keyword.

In [54]: u = pd.Series(['b', 'd', 'a', 'c'], index=[1, 3, 0, 2])

In [55]: s
Out[55]: 
0    a
1    b
2    c
3    d
dtype: object

In [56]: u
Out[56]: 
1    b
3    d
0    a
2    c
dtype: object

In [57]: s.str.cat(u)
Out[57]: 
0    ab
1    bd
2    ca
3    dc
dtype: object

In [58]: s.str.cat(u, join='left')
Out[58]: 
0    aa
1    bb
2    cc
3    dd
dtype: object

Warning

If the join keyword is not passed, the method cat() will currently fall back to the behavior before version 0.23.0 (i.e. no alignment), but a FutureWarning will be raised if any of the involved indexes differ, since this default will change to join='left' in a future version.

The usual options are available for join (one of 'left', 'outer', 'inner', 'right'). In particular, alignment also means that the different lengths do not need to coincide anymore.

In [59]: v = pd.Series(['z', 'a', 'b', 'd', 'e'], index=[-1, 0, 1, 3, 4])

In [60]: s
Out[60]: 
0    a
1    b
2    c
3    d
dtype: object

In [61]: v
Out[61]: 
-1    z
 0    a
 1    b
 3    d
 4    e
dtype: object

In [62]: s.str.cat(v, join='left', na_rep='-')
Out[62]: 
0    aa
1    bb
2    c-
3    dd
dtype: object

In [63]: s.str.cat(v, join='outer', na_rep='-')
Out[63]: 
-1    -z
 0    aa
 1    bb
 2    c-
 3    dd
 4    -e
dtype: object

The same alignment can be used when others is a DataFrame:

In [64]: f = d.loc[[3, 2, 1, 0], :]

In [65]: s
Out[65]: 
0    a
1    b
2    c
3    d
dtype: object

In [66]: f
Out[66]: 
     0  1
3    d  d
2  NaN  c
1    b  b
0    a  a

In [67]: s.str.cat(f, join='left', na_rep='-')
Out[67]: 
0    aaa
1    bbb
2    c-c
3    ddd
dtype: object

Concatenating a Series and many objects into a Series

Several array-like items (specifically: Series, Index, and 1-dimensional variants of np.ndarray) can be combined in a list-like container (including iterators, dict-views, etc.).

In [68]: s
Out[68]: 
0    a
1    b
2    c
3    d
dtype: object

In [69]: u
Out[69]: 
1    b
3    d
0    a
2    c
dtype: object

In [70]: s.str.cat([u, u.to_numpy()], join='left')
Out[70]: 
0    aab
1    bbd
2    cca
3    ddc
dtype: object

All elements without an index (e.g. np.ndarray) within the passed list-like must match in length to the calling Series (or Index), but Series and Index may have arbitrary length (as long as alignment is not disabled with join=None):

In [71]: v
Out[71]: 
-1    z
 0    a
 1    b
 3    d
 4    e
dtype: object

In [72]: s.str.cat([v, u, u.to_numpy()], join='outer', na_rep='-')
Out[72]: 
-1    -z--
 0    aaab
 1    bbbd
 2    c-ca
 3    dddc
 4    -e--
dtype: object

If using join='right' on a list-like of others that contains different indexes, the union of these indexes will be used as the basis for the final concatenation:

In [73]: u.loc[[3]]
Out[73]: 
3    d
dtype: object

In [74]: v.loc[[-1, 0]]
Out[74]: 
-1    z
 0    a
dtype: object

In [75]: s.str.cat([u.loc[[3]], v.loc[[-1, 0]]], join='right', na_rep='-')
Out[75]: 
-1    --z
 0    a-a
 3    dd-
dtype: object

Indexing with .str

You can use [] notation to directly index by position locations. If you index past the end of the string, the result will be a NaN.

In [76]: s = pd.Series(['A', 'B', 'C', 'Aaba', 'Baca', np.nan,
   ....:                'CABA', 'dog', 'cat'])
   ....: 

In [77]: s.str[0]
Out[77]: 
0      A
1      B
2      C
3      A
4      B
5    NaN
6      C
7      d
8      c
dtype: object

In [78]: s.str[1]
Out[78]: 
0    NaN
1    NaN
2    NaN
3      a
4      a
5    NaN
6      A
7      o
8      a
dtype: object

Extracting Substrings

Extract first match in each subject (extract)

Warning

In version 0.18.0, extract gained the expand argument. When expand=False it returns a Series, Index, or DataFrame, depending on the subject and regular expression pattern (same behavior as pre-0.18.0). When expand=True it always returns a DataFrame, which is more consistent and less confusing from the perspective of a user. expand=True is the default since version 0.23.0.

The extract method accepts a regular expression with at least one capture group.

Extracting a regular expression with more than one group returns a DataFrame with one column per group.

In [79]: pd.Series(['a1', 'b2', 'c3']).str.extract(r'([ab])(\d)', expand=False)
Out[79]: 
     0    1
0    a    1
1    b    2
2  NaN  NaN

Elements that do not match return a row filled with NaN. Thus, a Series of messy strings can be “converted” into a like-indexed Series or DataFrame of cleaned-up or more useful strings, without necessitating get() to access tuples or re.match objects. The dtype of the result is always object, even if no match is found and the result only contains NaN.

Named groups like

In [80]: pd.Series(['a1', 'b2', 'c3']).str.extract(r'(?P<letter>[ab])(?P<digit>\d)',
   ....:                                           expand=False)
   ....: 
Out[80]: 
  letter digit
0      a     1
1      b     2
2    NaN   NaN

and optional groups like

In [81]: pd.Series(['a1', 'b2', '3']).str.extract(r'([ab])?(\d)', expand=False)
Out[81]: 
     0  1
0    a  1
1    b  2
2  NaN  3

can also be used. Note that any capture group names in the regular expression will be used for column names; otherwise capture group numbers will be used.

Extracting a regular expression with one group returns a DataFrame with one column if expand=True.

In [82]: pd.Series(['a1', 'b2', 'c3']).str.extract(r'[ab](\d)', expand=True)
Out[82]: 
     0
0    1
1    2
2  NaN

It returns a Series if expand=False.

In [83]: pd.Series(['a1', 'b2', 'c3']).str.extract(r'[ab](\d)', expand=False)
Out[83]: 
0      1
1      2
2    NaN
dtype: object

Calling on an Index with a regex with exactly one capture group returns a DataFrame with one column if expand=True.

In [84]: s = pd.Series(["a1", "b2", "c3"], ["A11", "B22", "C33"])

In [85]: s
Out[85]: 
A11    a1
B22    b2
C33    c3
dtype: object

In [86]: s.index.str.extract("(?P<letter>[a-zA-Z])", expand=True)
Out[86]: 
  letter
0      A
1      B
2      C

It returns an Index if expand=False.

In [87]: s.index.str.extract("(?P<letter>[a-zA-Z])", expand=False)
Out[87]: Index(['A', 'B', 'C'], dtype='object', name='letter')

Calling on an Index with a regex with more than one capture group returns a DataFrame if expand=True.

In [88]: s.index.str.extract("(?P<letter>[a-zA-Z])([0-9]+)", expand=True)
Out[88]: 
  letter   1
0      A  11
1      B  22
2      C  33

It raises ValueError if expand=False.

>>> s.index.str.extract("(?P<letter>[a-zA-Z])([0-9]+)", expand=False)
ValueError: only one regex group is supported with Index

The table below summarizes the behavior of extract(expand=False) (input subject in first column, number of groups in regex in first row)

1 group

>1 group

Index

Index

ValueError

Series

Series

DataFrame

Extract all matches in each subject (extractall)

New in version 0.18.0.

Unlike extract (which returns only the first match),

In [89]: s = pd.Series(["a1a2", "b1", "c1"], index=["A", "B", "C"])

In [90]: s
Out[90]: 
A    a1a2
B      b1
C      c1
dtype: object

In [91]: two_groups = '(?P<letter>[a-z])(?P<digit>[0-9])'

In [92]: s.str.extract(two_groups, expand=True)
Out[92]: 
  letter digit
A      a     1
B      b     1
C      c     1

the extractall method returns every match. The result of extractall is always a DataFrame with a MultiIndex on its rows. The last level of the MultiIndex is named match and indicates the order in the subject.

In [93]: s.str.extractall(two_groups)
Out[93]: 
        letter digit
  match             
A 0          a     1
  1          a     2
B 0          b     1
C 0          c     1

When each subject string in the Series has exactly one match,

In [94]: s = pd.Series(['a3', 'b3', 'c2'])

In [95]: s
Out[95]: 
0    a3
1    b3
2    c2
dtype: object

then extractall(pat).xs(0, level='match') gives the same result as extract(pat).

In [96]: extract_result = s.str.extract(two_groups, expand=True)

In [97]: extract_result
Out[97]: 
  letter digit
0      a     3
1      b     3
2      c     2

In [98]: extractall_result = s.str.extractall(two_groups)

In [99]: extractall_result
Out[99]: 
        letter digit
  match             
0 0          a     3
1 0          b     3
2 0          c     2

In [100]: extractall_result.xs(0, level="match")
Out[100]: 
  letter digit
0      a     3
1      b     3
2      c     2

Index also supports .str.extractall. It returns a DataFrame which has the same result as a Series.str.extractall with a default index (starts from 0).

New in version 0.19.0.

In [101]: pd.Index(["a1a2", "b1", "c1"]).str.extractall(two_groups)
Out[101]: 
        letter digit
  match             
0 0          a     1
  1          a     2
1 0          b     1
2 0          c     1

In [102]: pd.Series(["a1a2", "b1", "c1"]).str.extractall(two_groups)
Out[102]: 
        letter digit
  match             
0 0          a     1
  1          a     2
1 0          b     1
2 0          c     1

Testing for Strings that Match or Contain a Pattern

You can check whether elements contain a pattern:

In [103]: pattern = r'[0-9][a-z]'

In [104]: pd.Series(['1', '2', '3a', '3b', '03c']).str.contains(pattern)
Out[104]: 
0    False
1    False
2     True
3     True
4     True
dtype: bool

Or whether elements match a pattern:

In [105]: pd.Series(['1', '2', '3a', '3b', '03c']).str.match(pattern)
Out[105]: 
0    False
1    False
2     True
3     True
4    False
dtype: bool

The distinction between match and contains is strictness: match relies on strict re.match, while contains relies on re.search.

Methods like match, contains, startswith, and endswith take an extra na argument so missing values can be considered True or False:

In [106]: s4 = pd.Series(['A', 'B', 'C', 'Aaba', 'Baca', np.nan, 'CABA', 'dog', 'cat'])

In [107]: s4.str.contains('A', na=False)
Out[107]: 
0     True
1    False
2    False
3     True
4    False
5    False
6     True
7    False
8    False
dtype: bool

Creating Indicator Variables

You can extract dummy variables from string columns. For example if they are separated by a '|':

In [108]: s = pd.Series(['a', 'a|b', np.nan, 'a|c'])

In [109]: s.str.get_dummies(sep='|')
Out[109]: 
   a  b  c
0  1  0  0
1  1  1  0
2  0  0  0
3  1  0  1

String Index also supports get_dummies which returns a MultiIndex.

New in version 0.18.1.

In [110]: idx = pd.Index(['a', 'a|b', np.nan, 'a|c'])

In [111]: idx.str.get_dummies(sep='|')
Out[111]: 
MultiIndex(levels=[[0, 1], [0, 1], [0, 1]],
           codes=[[1, 1, 0, 1], [0, 1, 0, 0], [0, 0, 0, 1]],
           names=['a', 'b', 'c'])

See also get_dummies().

Method Summary

Method

Description

cat()

Concatenate strings

split()

Split strings on delimiter

rsplit()

Split strings on delimiter working from the end of the string

get()

Index into each element (retrieve i-th element)

join()

Join strings in each element of the Series with passed separator

get_dummies()

Split strings on the delimiter returning DataFrame of dummy variables

contains()

Return boolean array if each string contains pattern/regex

replace()

Replace occurrences of pattern/regex/string with some other string or the return value of a callable given the occurrence

repeat()

Duplicate values (s.str.repeat(3) equivalent to x * 3)

pad()

Add whitespace to left, right, or both sides of strings

center()

Equivalent to str.center

ljust()

Equivalent to str.ljust

rjust()

Equivalent to str.rjust

zfill()

Equivalent to str.zfill

wrap()

Split long strings into lines with length less than a given width

slice()

Slice each string in the Series

slice_replace()

Replace slice in each string with passed value

count()

Count occurrences of pattern

startswith()

Equivalent to str.startswith(pat) for each element

endswith()

Equivalent to str.endswith(pat) for each element

findall()

Compute list of all occurrences of pattern/regex for each string

match()

Call re.match on each element, returning matched groups as list

extract()

Call re.search on each element, returning DataFrame with one row for each element and one column for each regex capture group

extractall()

Call re.findall on each element, returning DataFrame with one row for each match and one column for each regex capture group

len()

Compute string lengths

strip()

Equivalent to str.strip

rstrip()

Equivalent to str.rstrip

lstrip()

Equivalent to str.lstrip

partition()

Equivalent to str.partition

rpartition()

Equivalent to str.rpartition

lower()

Equivalent to str.lower

casefold()

Equivalent to str.casefold

upper()

Equivalent to str.upper

find()

Equivalent to str.find

rfind()

Equivalent to str.rfind

index()

Equivalent to str.index

rindex()

Equivalent to str.rindex

capitalize()

Equivalent to str.capitalize

swapcase()

Equivalent to str.swapcase

normalize()

Return Unicode normal form. Equivalent to unicodedata.normalize

translate()

Equivalent to str.translate

isalnum()

Equivalent to str.isalnum

isalpha()

Equivalent to str.isalpha

isdigit()

Equivalent to str.isdigit

isspace()

Equivalent to str.isspace

islower()

Equivalent to str.islower

isupper()

Equivalent to str.isupper

istitle()

Equivalent to str.istitle

isnumeric()

Equivalent to str.isnumeric

isdecimal()

Equivalent to str.isdecimal

Scroll To Top