converting a pandas date to week number

Just access the dt week attribute:

In [286]:
df['Date'].dt.week

Out[286]:
0    25
dtype: int64

In [287]:
df['Week_Number'] = df['Date'].dt.week
df

Out[287]:
        Date  Week_Number
0 2015-06-17           25

Here is another possibility using strftime. strftime.org is a good resource.

df['Week_Number'] = df['Date'].dt.strftime('%U')

'%U' represents the week number of the year (Sunday as the first day of the week) as a zero padded decimal number. All days in a new year preceding the first Sunday are considered to be in week 0.

If you have dates from multiple years, I recommend creating a Year-Week combination

df['Year-Week'] = df['Date'].dt.strftime('%Y-%U')

Pandas has its .dayofyear and .weekofyear functionality, which can be applied straight away to the output of pandas.to_datetime(df['column_name']), giving type "Timestamp" as the output.

import pandas as pd
df['formatted_date'] = pd.to_datetime(df['datetime'])
df['day_of_year'] = df.formatted_date.apply(lambda x: x.dayofyear)
df['week_of_year'] = df.formatted_date.apply(lambda x: x.weekofyear)

from datetime import date
df_date = pd.DataFrame([date.today()],columns  = ['today'])
print(df_date)
#### Print Output ####
#        today
#0  2019-09-07
df_date['weeknum'] = df_date.today.apply(lambda x:x.isocalendar()[1])
print(df_date)
#### Print Output ####
#        today  weeknum
#0  2019-09-07       36