Resample from monthly to weekly works, but not vice versa

[JaneTan]

Hi

I have a function that works OK when I tried to re-sample from weekly to monthly data. But it doesn't work when I re-sample from monthly to weekly data. Appreciate if someone could assist

Thank you

df=pd.read_excel(input_file, sheet_name='weekly', usecols='A:D', na_values='ND', index_col=0, header=0)
#df=pd.read_excel(input_file, sheet_name='mthly', usecols='A:D', na_values='ND', index_col=0, header=0)
df.index.names = ['Period']
df.index = pd.to_datetime(df.index)

def CONVERT(series, freq, obs, technique='CONSTANT', ign='OFF'):
    if technique=='CONSTANT' and ign=='ON':
        if  obs=='BEGINNING' :
            q= pd.Series(df[series], index=df.index)
            groups =q.resample(freq)
            val = [x[1][0] for x in groups]
            m=pd.Series(val, groups.indices)
        return m

print(CONVERT('Values','M',  'BEGINNING', 'CONSTANT', 'ON'))

WEEKLY INPUT DATA

Period Values
20-Jan-89 3
27-Jan-89 4
3-Feb-89 2
10-Feb-89 5
17-Feb-89 3
24-Feb-89 5
3-Mar-89 5
10-Mar-89 6
17-Mar-89 4
24-Mar-89 5
31-Mar-89 7
7-Apr-89 6
14-Apr-89 6
21-Apr-89 4
28-Apr-89 7
5-May-89 5
12-May-89 6
19-May-89 5
26-May-89 7
2-Jun-89 7

Output from code

1989-01-31 3
1989-02-28 2
1989-03-31 5
1989-04-30 6
1989-05-31 5
1989-06-30 7

print(CONVERT('Values','W',  'BEGINNING', 'CONSTANT', 'ON'))

MONTHLY INPUT DATA
Period Values
Jan1989 1
Feb1989 3
Mar1989 7
Apr1989 5
May1989 4

OUTPUT ERROR

IndexError: index out of bounds