How to group dataframe rows into list in pandas groupby

Question

I have a pandas data frame df like   a b A 1 A 2 B 5 B 5 B 4 C 6   I want to group by the first column and get second column as lists in rows   A  1 2  B  5 5 4  C  6    Is it possible to do something like this using pandas groupby

User · Answer

Here I have grouped elements with     as a separator      import pandas as pd      df   pd read csv  input csv        df     Out 1         Area  Keywords     0  A  1     1  A  2     2  B  5     3  B  5     4  B  4     5  C  6      df dropna inplace    True      df  Area   df  Area   apply lambda x x lower   strip        print df columns     df op   df groupby  Area   agg   Keywords  lambda x       join x         df op to csv  output csv       Out 2       df op     Area  Keywords      A        1  2      B     5  5  4      C           6

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Building upon  B M answer  here is a more general version and updated to work with newer library version   numpy version 1 19 2  pandas version 1 2 1  And this solution can also deal with multi-indices  However this is not heavily tested  use with caution  If performance is important go down to numpy level  import pandas as pd import numpy as np  np random seed 0  df   pd DataFrame   a   np random randint 0  10  90    b    1 2 3  30   c  list  abcefghij   10   d   list  hij   30     def f multi df col names       if not isinstance col names list           col names    col names               values   df sort values col names  values T      col idcs    df columns get loc cn  for cn in col names      other col names    name for idx  name in enumerate df columns  if idx not in col idcs      other col idcs    df columns get loc cn  for cn in other col names         split df into indexing colums  keys  and data colums  vals      keys   values col idcs        vals   values other col idcs               list of tuple of key pairs     multikeys   list zip  keys              remember unique key pairs and ther indices     ukeys  index   np unique multikeys  return index True  axis 0             split data columns according to those indices     arrays   np split vals  index 1    axis 1         resulting list of subarrays has same number of subarrays as unique key pairs       each subarray has the following shape           rows   number of non-grouped data columns          cols   number of data points grouped into that unique key pair            prepare multi index     idx   pd MultiIndex from arrays ukeys T  names col names        list agg vals   dict       for tup in zip  arrays  other col names           col vals   tup  -1    first entries are the subarrays from above          col name   tup -1     last entry is data-column name                  list agg vals col name    col vals      df2   pd DataFrame data list agg vals  index idx      return df2  Tests  In  227    timeit f multi df    a   d     2 54 ms    64 7   s per loop  mean    std  dev  of 7 runs  100 loops each   In  228    timeit df groupby   a   d    agg list   4 56 ms    61 5   s per loop  mean    std  dev  of 7 runs  100 loops each     Results  for the random seed 0 one would get

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If performance is important go down to numpy level   import numpy as np  df   pd DataFrame   a   np random randint 0  60  600    b    1  2  5  5  4  6  100    def f df            keys  values   df sort values  a   values T          ukeys  index   np unique keys  True           arrays   np split values  index 1             df2   pd DataFrame   a  ukeys   b   list a  for a in arrays             return df2   Tests   In  301    timeit f df  1000 loops  best of 3  1 64 ms per loop  In  302    timeit df groupby  a    b   apply list  100 loops  best of 3  5 26 ms per loop

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Use any of the following groupby and agg recipes     Setup df   pd DataFrame      a     A    A    B    B    B    C       b    1  2  5  5  4  6      c     x    y    z    x    y    z      df     a  b  c 0  A  1  x 1  A  2  y 2  B  5  z 3  B  5  x 4  B  4  y 5  C  6  z   To aggregate multiple columns as lists  use any of the following   df groupby  a   agg list  df groupby  a   agg pd Series tolist              b          c a                       A      1  2       x  y  B   5  5  4    z  x  y  C         6          z      To group-listify a single column only  convert the groupby to a SeriesGroupBy object  then call SeriesGroupBy agg  Use   df groupby  a   agg   b   list      4 42 ms  df groupby  a    b   agg list       2 76 ms - faster  a A        1  2  B     5  5  4  C           6  Name  b  dtype  object

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Answer based on  EdChum s comment on his answer  Comment is this - groupby is notoriously slow and memory hungry  what you could do is sort by column A  then find the idxmin and idxmax  probably store this in a dict  and use this to slice your dataframe would be faster I think   Let s first create a dataframe with 500k categories in first column and total df shape 20 million as mentioned in question  df   pd DataFrame columns   a    b    df  a      np random randint low 0  high 500000  size  20000000     astype str  df  b     list range 20000000   print df shape  df head      Sort data by first column  df sort values by   a    ascending True  inplace True  df reset index drop True  inplace True     Create a temp column df  temp idx     list range df shape 0       Take all values of b in a separate list all values b   list df b values  print len all values b      For each category in column a  find min and max indexes gp df   df groupby   a    agg   temp idx    np min  np max    gp df reset index inplace True  gp df columns     a    temp idx min    temp idx max      Now create final list b column  using min and max indexes for each category of a and filtering list of b   gp df  list b     gp df   temp idx min    temp idx max    apply lambda x  all values b x 0  x 1  1   axis 1   print gp df shape  gp df head    This above code takes 2 minutes for 20 million rows and 500k categories in first column

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It is time to use agg instead of apply    When  df   pd DataFrame    a    A   A   B   B   B   C     b   1 2 5 5 4 6    c    1 2 5 5 4 6      If you want multiple columns stack into list   result in pd DataFrame  df groupby  a     b    c    agg list    or  df groupby  a   agg list    If you want single column in list  result in ps Series  df groupby  a    b   agg list   or df groupby  a    b   apply list    Note  result in pd DataFrame is about 10x slower than result in ps Series when you only aggregate single column  use it in multicolumns case

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As you were saying the groupby method of a pd DataFrame object can do the job   Example   L     A   A   B   B   B   C    N    1 2 5 5 4 6    import pandas as pd  df   pd DataFrame zip L N  columns   list  LN       groups   df groupby df L    groups groups         A    0  1    B    2  3  4    C    5     which gives and index-wise description of the groups   To get elements of single groups  you can do  for instance   groups get group  A         L  N   0  A  1   1  A  2    groups get group  B         L  N   2  B  5   3  B  5   4  B  4

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Let us using df groupby with list and Series constructor   pd Series  x   y b tolist   for x   y in df groupby  a     Out 664    A        1  2  B     5  5  4  C           6  dtype  object

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The easiest way I have see no achieve most of the same thing at least for one column which is similar to Anamika s answer just with the tuple syntax for the aggregate function   df groupby  a   agg b   b   unique    c   c   unique

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A handy way to achieve this would be   df groupby  a   agg   b  lambda x  list x      Look into writing Custom Aggregations  https   www kaggle com akshaysehgal how-to-group-by-aggregate-using-py

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If looking for a unique list while grouping multiple columns this could probably help   df groupby  a   agg lambda x  list set x    reset index

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To solve this for several columns of a dataframe   In  5   df   pd DataFrame    a    A   A   B   B   B   C     b   1 2 5 5 4 6   c            3 3 3 4 4 4     In  6   df Out 6       a  b  c 0  A  1  3 1  A  2  3 2  B  5  3 3  B  5  4 4  B  4  4 5  C  6  4  In  7   df groupby  a   agg lambda x  list x   Out 7               b          c a                       A      1  2       3  3  B   5  5  4    3  4  4  C         6          4    This answer was inspired from Anamika Modi s answer  Thank you

User · Answer

You can do this using groupby to group on the column of interest and then apply list to every group   In  1   df   pd DataFrame    a    A   A   B   B   B   C     b   1 2 5 5 4 6            df  Out 1       a  b 0  A  1 1  A  2 2  B  5 3  B  5 4  B  4 5  C  6  In  2   df groupby  a    b   apply list  Out 2    a A        1  2  B     5  5  4  C           6  Name  b  dtype  object  In  3   df1   df groupby  a    b   apply list  reset index name  new           df1 Out 3       a        new 0  A      1  2  1  B   5  5  4  2  C         6

[python] How to group dataframe rows into list in pandas groupby

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