df=pd.DataFrame(data=[[301,301,302,303],[['a'],['b','c'],['e','f',33,'Z'],42],index=['id','foo']).T

id     foo
0  301     [a]
1  301  [b, c]
2  302  [e, f,33,'Z']
3  303   42

id     foo
0  301     a
1  301     b
1  301     c
2  302     e
2  302     f
2  302     33
2  302     Z
3  303     42

df['foo']  = [x if isinstance(x, list) else [x] for x in df['foo']]

from itertools import chain

df = pd.DataFrame({
    'id' : df['id'].repeat(df['foo'].str.len()),
    'foo' : list(chain.from_iterable(df['foo'].tolist()))

})
print (df)
    id foo
0  301   a
1  301   b
1  301   c
2  302   e
2  302   f
2  302  33
2  302   Z
3  303  42

df.set_index('id')['foo'].apply(pd.Series).stack().reset_index(name='foo').drop('level_1', axis=1)

id foo
0  301   a
1  301   b
2  301   c
3  302   e
4  302   f
5  302  33
6  302   Z
7  303  42

id test            foo
0  301    1            [a]
1  301    2         [b, c]
2  302    3  [e, f, 33, Z]
3  303    4             42

df.set_index(['id','test'])['foo'].apply(pd.Series).stack().reset_index(name='foo').drop('level_2', axis=1)

id  test foo
0  301     1   a
1  301     2   b
2  301     2   c
3  302     3   e
4  302     3   f
5  302     3  33
6  302     3   Z
7  303     4  42