pandas or python equivalent of tidyr complete

You can use reindex by MultiIndex.from_product:

df = df.set_index(['user','x'])
mux = pd.MultiIndex.from_product([df.index.levels[0], df.index.levels[1]],names=['user','x'])
df = df.reindex(mux, fill_value=0).reset_index()
print (df)
    user  x  y
0      1  a  1
1      1  b  1
2      1  c  0
3      1  d  0
4      2  a  1
5      2  b  0
6      2  c  0
7      2  d  0
8      3  a  1
9      3  b  0
10     3  c  1
11     3  d  1

Or set_index + stack + unstack:

df = df.set_index(['user','x'])['y'].unstack(fill_value=0).stack().reset_index(name="y")
print (df)
    user  x  y
0      1  a  1
1      1  b  1
2      1  c  0
3      1  d  0
4      2  a  1
5      2  b  0
6      2  c  0
7      2  d  0
8      3  a  1
9      3  b  0
10     3  c  1
11     3  d  1