Right way to reverse a pandas DataFrame

Question

Here is my code   import pandas as pd  data   pd DataFrame   Odd   1 3 5 6 7 9    Even   0 2 4 6 8 10     for i in reversed data       print data  Odd    data  Even      When I run this code  i get the following error   Traceback  most recent call last     File  C  Python33 lib site-packages pandas core generic py   line 665  in  get item cache     return cache item  KeyError  5  During handling of the above exception  another exception occurred   Traceback  most recent call last     File  C  Users       Documents                      py   line 5  in  lt module gt      for i in reversed data     File  C  Python33 lib site-packages pandas core frame py   line 2003  in   getitem       return self  get item cache key    File  C  Python33 lib site-packages pandas core generic py   line 667  in  get item cache     values   self  data get item    File  C  Python33 lib site-packages pandas core internals py   line 1656  in get        block   self  find block item    File  C  Python33 lib site-packages pandas core internals py   line 1936  in  find block     self  check have item    File  C  Python33 lib site-packages pandas core internals py   line 1943  in  check have     raise KeyError  no item named  s    com pprint thing item   KeyError   no item named 5    Why am I getting this error  How can I fix that  What is the right way to reverse pandas DataFrame

User · Answer

One way to do this if dealing with sorted range index is:

data = data.sort_index(ascending=False)

This approach has the benefits of (1) being a single line, (2) not requiring a utility function, and most importantly (3) not actually changing any of the data in the dataframe.

Caveat: this works by sorting the index in descending order and so may not always be appropriate or generalize for any given Dataframe.

User · Answer

None of the existing answers resets the index after reversing the dataframe    For this  do the following     data   -1  reset index     Here s a utility function that also removes the old index column  as per  Tim s comment    def reset my index df     res   df   -1  reset index drop True    return res    Simply pass your dataframe into the function

User · Answer

This works       for i r in data   -1  iterrows            print r  Odd    r  Even

User · Answer

You can reverse the rows in an even simpler way   df   -1

User · Answer

data reindex index data index   -1     or simply   data iloc   -1    will reverse your data frame  if you want to have a for loop which goes from down to up you may do   for idx in reversed data index       print idx  data loc idx   Even    data loc idx   Odd      or  for idx in reversed data index       print idx  data Even idx   data Odd idx     You are getting an error because reversed first calls data   len     which returns 6  Then it tries to call data j - 1  for j in range 6  0  -1   and the first call would be data 5   but in pandas dataframe data 5  means column 5  and there is no column 5 so it will throw an exception    see docs

User · Answer

What is the right way to reverse a pandas DataFrame   TL DR  df   -1  This is objectively IMO the best method for reversing a DataFrame  because it is a ONE step operation  also very readable  assuming familiarity with slice notation    Long Version I ve found the ol  slicing trick df   -1   or the equivalent df loc   -1 1  to be the most concise and idiomatic way of reversing a DataFrame  This mirrors the python list reversal syntax lst   -1  and is clear in its intent   With the loc syntax  you are also able to slice columns if required  so it is a bit more flexible  Some points to consider while handling the index    quot what if I want to reverse the index as well  quot   you re already done  df   -1  reverses both the index and values     quot what if I want to drop the index from the result  quot   you can call  reset index drop True  at the end     quot what if I want to keep the index untouched  IOW  only reverse the data  not the index   quot   this is somewhat unconventional because it implies the index isn t really relevant to the data  Perhaps consider removing it entirely  Although what you re asking for can technically be achieved using either df      df   -1  which creates an in-place update to df  or df loc   -1  set index df index   which returns a copy      1  df loc   -1  and df iloc   -1  are equivalent since the slicing syntax remains the same  whether you re reversing by position  iloc  or label  loc    The Proof is in the Pudding  X-axis represents the dataset size  Y-axis represents time taken to reverse  No method scales as well as the slicing trick  it s all the way at the bottom of the graph  Benchmarking code for reference  plots generated using perfplot   Comments on other solutions  df reindex index df index   -1   is clearly a popular solution  but on first glance  how obvious is it to an unfamiliar reader that this code is  quot reversing a DataFrame quot   Additionally  this is reversing the index  then using that intermediate result to reindex  so this is essentially a TWO step operation  when it could ve been just one    df sort index ascending False  may work in most cases where you have a simple range index  but this assumes your index was sorted in ascending order and so doesn t generalize well   PLEASE do not use iterrows  I see some options suggesting iterating in reverse  Whatever your use case  there is likely a vectorized method available  but if there isn t then you can use something a little more reasonable such as list comprehensions  See How to iterate over rows in a DataFrame in Pandas for more detail on why iterrows is an antipattern

[python] Right way to reverse a pandas DataFrame?

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