Python Pandas: Counting the Frequency of unique values over all Columns

Question

I have a question, how does one count the number of unique values that occur within each column of a pandas data-frame?

Say I have a data frame named df that looks like this:

1    2    3    4    
a    yes  f    c
b    no   f    e
c    yes  d    h

I am wanting to get output that shows the frequency of unique values within the four columns. The output would be something similar to this:

Column    # of Unique Values
 1          3
 2          2
 3          2
 4          3

I don't need to know what the unique values are, just how many there are within each column.

I have played around with something like this:

df[all_cols].value_counts()

[all_cols] is a list of all the columns within the data frame. But this is counting how many times the value appears within the column.

Any advice/suggestions would be a great help. Thanks

Answer 1

You could apply Series.nunique :

>>> df.apply(pd.Series.nunique)
1    3
2    2
3    2
4    3
dtype: int64

Or you could do a groupby/nunique on the unstacked version of the frame:

>>> df.unstack().groupby(level=0).nunique()
1    3
2    2
3    2
4    3
dtype: int64

Both of these produce a Series, which you could then use to build a frame with whatever column names you wanted.

Answer 2

You could try df.nunique()

>>> df.nunique()
1    3
2    2
3    2
4    3
dtype: int64