How to group a dataframe and summarize over subgroups of consecutive numbers in Python?

Question

I have a dataframe with a column containing ids and other column containing numbers:

df1 = {'ID':[400, 400, 400, 400, 400, 400, 500, 500, 500, 500], 
      'Number':[1, 2, 3, 4, 8, 9, 22, 23, 26, 27]}

You may note that each Id has their correponding series of consecutive numbers in the column "Number". For example:

Id 400 contains a series of length 4 {1, 2, 3, 4} and another of length 2 {8, 9}

I´d like to obtain for each Id, the average length of their corresponding series. In this example:

df2 = {'ID':[400, 500], 'avg_length':[3, 2]}

Any ideas will be much appreciated!

Answer 1

Here is one way, uses groupby twice,

df1['tmp'] = (df1.Number - df1.Number.shift() > 1).cumsum()

df1.groupby(['ID', 'tmp']).Number.count().groupby(level = 0).mean().reset_index(name = 'avg_length')

2.29 ms ± 75.8 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

    ID  avg_length
0   400 3
1   500 2

Option 2: Without using apply twice, still uses tmp column created earlier

df1.groupby('ID').tmp.apply(lambda x: x.value_counts().mean()).reset_index(name = 'avg_length')

2.25 ms ± 99.3 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)