Here's the phony data:

df = pd.DataFrame({'email': ['u1','u1','u1','u2','u2','u2'],'timestamp': [3, 1, 5, 11, 15, 9]})

What I intend to retrieve is the time diff in each group of email. Thus, after sorting by timestamp in each group, the data should be:

u1  5
u1  3
u1  1
u2  15
u2  11
u2  9

the result should be:

u1  2  # 5-3
u1  2  # 3-1
u2  4  # 15-11
u2  2  # 11-9

Could anyone tell me what I should do next? Great thanks.

df = pd.DataFrame({'email': ['u1','u1','u1','u2','u2','u2'],'timestamp': [3, 1, 5, 11, 15, 9]})(df.sort_values(['email', 'timestamp'], ascending=[True, False]).groupby('email')['timestamp'].diff(-1).dropna())
Out: 
2    2.0
0    2.0
4    4.0
3    2.0
Name: timestamp, dtype: float64

To keep the email column:

df.sort_values(['email', 'timestamp'], ascending=[True, False], inplace=True)
df.assign(diff=df.groupby('email')['timestamp'].diff(-1)).dropna()
Out: email  timestamp  diff
2    u1          5   2.0
0    u1          3   2.0
4    u2         15   4.0
3    u2         11   2.0

If you don't want the timestamp column you can add .drop('timestamp', axis=1) at the end.