Python Pandas: Keeping only dataframe rows containing first occurrence of an item

Question

I have this:

    Date value
0   1975     a
21  1975     b
1   1976     b
22  1976     c
3   1977     a
2   1977     b
4   1978     c
25  1978     d
5   1979     e
26  1979     f
6   1980     a
27  1980     f

I am having trouble finding a way to keep only the lines containing the first occurrence of a 'value'. I want to drop duplicate 'values', keeping the row with the lowest 'Date'.The end result should be:

    Date value
0   1975     a
21  1975     b
22  1976     c
25  1978     d
5   1979     e
26  1979     f

Answer 1

To make a bit more explicit what Quazi posted: drop_duplicates() is what you need. By default, it keeps the first occurence and drops everything thereafter - look at the manual for more information. So, to be sure, you should do

>>> dataframe = oldDf.sort('Date').drop_duplicates(subset=['value'])
>>> dataframe
Out[490]: 
    Date value
0   1975     a
21  1975     b
22  1976     c
25  1978     d
5   1979     e
26  1979     f

Answer 2

df.drop_duplicates(subset=['value'], inplace=True)