Sorting date values in a dataframe doesn't work

Question

I have the column 'Created At' in this form:

The date is in this format: '%d/%m/%Y' -> day, month, year

obj = {'Created At': ['01/01/2017', '01/02/2017', '02/01/2017', 
                      '02/02/2017', 
                      '03/01/2017', '03/02/2017','04/01/2017' ], 
       'Text': [1, 70,14,17,84,76,32]}

df = pd.DataFrame(data=obj)

I did it, but dosen't work:

df.sort_values(by='Created At', inplace=True)

It seems that it sorts only the days and disregards the month. What do I do?

No it does sort it properly: only in case two dates are the same, it will sort on the month. That is because your dates are strings, and these are sorted lexicographically. — willeM_ Van Onsem
– willeM_ Van Onsem, Commented Dec 27, 2019 at 21:26

willeM_ Van Onsem · Accepted Answer · 2019-12-27 21:30:02Z

7

It does sort it properly: your dates are strings here. Strings are sorted lexicographically. So that means that only if the first character is the same, it will look at the second character, etc.

You therefore might want to convert the column first to datetime objects:

df['Created At'] = pd.to_datetime(df['Created At'], format='%d/%m/%Y')

then we can sort the dataframe, and obtain:

>>> df.sort_values(by='Created At', inplace=True)
>>> df
  Created At  Text
0 2017-01-01     1
2 2017-01-02    14
4 2017-01-03    84
6 2017-01-04    32
1 2017-02-01    70
3 2017-02-02    17
5 2017-02-03    76

answered Dec 27, 2019 at 21:30

willeM_ Van Onsem

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4 Comments

Gizelly Over a year ago

Sorry, but i have a quenstion: When you do it: format='%d/%m/%Y' The result is: Year, month, day? For exemple: 2017-02-02 2017-02-03

willeM_ Van Onsem Over a year ago

@Gizelly: the format specifies how we parse the strings to a datetime object. So once it is parsed, the datetime objects, are represented as yyyy-mm-dd. These are not strings anymore, but datetime objects. So it makes calculations, etc. on dates more convenient.

Gizelly Over a year ago

Wow! Thanks for teach me a new concept! I had no idea about lexicography. You helped me so much!

sancho.s ReinstateMonicaCellio Over a year ago

Missing inplace was the issue in my case.

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