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]: 
     a
     b
     c
  aaba
  baca
   NaN
  caba
   dog
   cat
dtype: object

In [3]: s.str.upper()
Out[3]: 
     A
     B
     C
  AABA
  BACA
   NaN
  CABA
   DOG
   CAT
dtype: object

In [4]: s.str.len()
Out[4]: 
  1.0
  1.0
  1.0
  4.0
  4.0
  NaN
  4.0
  3.0
  3.0
dtype: float64

In [5]: idx = pd.Index([' jack', 'jill ', ' jesse ', 'frank'])

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

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

In [8]: idx.str.rstrip()
Out[8]: Index([u' jack', u'jill', u' jesse', u'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(randn(3, 2), columns=[' Column A ', ' Column B '],
   ...:                   index=range(3))
   ...: 

In [10]: df
Out[10]: 
    Column A    Column B 
0    0.017428    0.039049
1   -2.240248    0.847859
2   -1.342107    0.368828

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

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

In [12]: df.columns.str.lower()
Out[12]: Index([u' column a ', u' 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, lowercasing 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  0.017428  0.039049
1 -2.240248  0.847859
2 -1.342107  0.368828

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 of 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

Easy to expand this to return a DataFrame using expand.

In [19]: s2.str.split('_', expand=True)
Out[19]: 
   1     2
  a     b     c
  c     d     e
NaN  None  None
  f     g     h

It is also possible to limit the number of splits:

In [20]: s2.str.split('_', expand=True, n=1)
Out[20]: 
   1
  a   b_c
  c   d_e
NaN  None
  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]: 
   1
a_b     c
c_d     e
NaN  None
f_g     h

Methods like replace and findall take regular expressions, too:

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

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

In [30]: s.str[0]
Out[30]: 
0      A
1      B
2      C
3      A
4      B
5    NaN
6      C
7      d
8      c
dtype: object

In [31]: s.str[1]
Out[31]: 
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)¶

New in version 0.13.0.

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.

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 [32]: pd.Series(['a1', 'b2', 'c3']).str.extract('([ab])(\d)', expand=False)
Out[32]: 
     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 [33]: pd.Series(['a1', 'b2', 'c3']).str.extract('(?P<letter>[ab])(?P<digit>\d)', expand=False)
Out[33]: 
  letter digit
0      a     1
1      b     2
2    NaN   NaN

and optional groups like

In [34]: pd.Series(['a1', 'b2', '3']).str.extract('([ab])?(\d)', expand=False)
Out[34]: 
     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 [35]: pd.Series(['a1', 'b2', 'c3']).str.extract('[ab](\d)', expand=True)
Out[35]: 
     0
0    1
1    2
2  NaN

It returns a Series if expand=False.

In [36]: pd.Series(['a1', 'b2', 'c3']).str.extract('[ab](\d)', expand=False)
Out[36]: 
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 [37]: s = pd.Series(["a1", "b2", "c3"], ["A11", "B22", "C33"])

In [38]: s
Out[38]: 
A11    a1
B22    b2
C33    c3
dtype: object

In [39]: s.index.str.extract("(?P<letter>[a-zA-Z])", expand=True)
Out[39]: 
  letter
0      A
1      B
2      C

It returns an Index if expand=False.

In [40]: s.index.str.extract("(?P<letter>[a-zA-Z])", expand=False)
Out[40]: Index([u'A', u'B', u'C'], dtype='object', name=u'letter')

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

In [41]: s.index.str.extract("(?P<letter>[a-zA-Z])([0-9]+)", expand=True)
Out[41]: 
  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 [42]: s = pd.Series(["a1a2", "b1", "c1"], index=["A", "B", "C"])

In [43]: s
Out[43]: 
A    a1a2
B      b1
C      c1
dtype: object

In [44]: two_groups = '(?P<letter>[a-z])(?P<digit>[0-9])'

In [45]: s.str.extract(two_groups, expand=True)
Out[45]: 
  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 [46]: s.str.extractall(two_groups)
Out[46]: 
        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 [47]: s = pd.Series(['a3', 'b3', 'c2'])

In [48]: s
Out[48]: 
0    a3
1    b3
2    c2
dtype: object

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

In [49]: extract_result = s.str.extract(two_groups, expand=True)

In [50]: extract_result
Out[50]: 
  letter digit
0      a     3
1      b     3
2      c     2

In [51]: extractall_result = s.str.extractall(two_groups)

In [52]: extractall_result
Out[52]: 
        letter digit
  match             
0 0          a     3
1 0          b     3
2 0          c     2

In [53]: extractall_result.xs(0, level="match")
Out[53]: 
  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 [54]: pd.Index(["a1a2", "b1", "c1"]).str.extractall(two_groups)
Out[54]: 
        letter digit
  match             
0 0          a     1
  1          a     2
1 0          b     1
2 0          c     1

In [55]: pd.Series(["a1a2", "b1", "c1"]).str.extractall(two_groups)
Out[55]: 
        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 [56]: pattern = r'[a-z][0-9]'

In [57]: pd.Series(['1', '2', '3a', '3b', '03c']).str.contains(pattern)
Out[57]: 
0    False
1    False
2    False
3    False
4    False
dtype: bool

or match a pattern:

In [58]: pd.Series(['1', '2', '3a', '3b', '03c']).str.match(pattern, as_indexer=True)
Out[58]: 
0    False
1    False
2    False
3    False
4    False
dtype: bool

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

Warning

In previous versions, match was for extracting groups, returning a not-so-convenient Series of tuples. The new method extract (described in the previous section) is now preferred.

This old, deprecated behavior of match is still the default. As demonstrated above, use the new behavior by setting as_indexer=True. In this mode, match is analogous to contains, returning a boolean Series. The new behavior will become the default behavior in a future release.

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

In [59]: s4 = pd.Series(['A', 'B', 'C', 'Aaba', 'Baca', np.nan, 'CABA', 'dog', 'cat'])

In [60]: s4.str.contains('A', na=False)
Out[60]: 
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 [61]: s = pd.Series(['a', 'a|b', np.nan, 'a|c'])

In [62]: s.str.get_dummies(sep='|')
Out[62]: 
   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 [63]: idx = pd.Index(['a', 'a|b', np.nan, 'a|c'])

In [64]: idx.str.get_dummies(sep='|')
Out[64]: 
MultiIndex(levels=[[0, 1], [0, 1], [0, 1]],
           labels=[[1, 1, 0, 1], [0, 1, 0, 0], [0, 0, 0, 1]],
           names=[u'a', u'b', u'c'])

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 with some other string
`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`
`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`

Table Of Contents

Search

Working with Text Data¶

Splitting and Replacing Strings¶

Indexing with `.str`¶

Extracting Substrings¶

Extract first match in each subject (extract)¶

Extract all matches in each subject (extractall)¶

Testing for Strings that Match or Contain a Pattern¶

Creating Indicator Variables¶

Method Summary¶

Table Of Contents

Search

Working with Text Data¶

Splitting and Replacing Strings¶

Indexing with .str¶

Extracting Substrings¶

Extract first match in each subject (extract)¶

Extract all matches in each subject (extractall)¶

Testing for Strings that Match or Contain a Pattern¶

Creating Indicator Variables¶

Method Summary¶

Indexing with `.str`¶