Check if pandas dataframe is subset of other dataframe

Question

I have two Python Pandas dataframes A, B, with the same columns (obviously with different data). I want to check A is a subset of B, that is, all rows of A are contained in

Answer 1

Method DataFrame.merge(another_DF) merges on the intersection of the columns by default (uses all columns with same names from both DFs) and uses how='inner' - so we expect to have the same # of rows after inner join (if neither of DFs has duplicates):

len(A.merge(B)) == len(A)

PS it will NOT work properly if one of DFs have duplicated rows - see below for such cases

Demo:

In [128]: A
Out[128]:
   A  B  C
0  1  2  3
1  4  5  6

In [129]: B
Out[129]:
   A  B  C
0  4  5  6
1  1  2  3
2  9  8  7

In [130]: len(A.merge(B)) == len(A)
Out[130]: True

---

for data sets containing duplicates, we can remove duplicates and use the same method:

In [136]: A
Out[136]:
   A  B  C
0  1  2  3
1  4  5  6
2  1  2  3

In [137]: B
Out[137]:
   A  B  C
0  4  5  6
1  1  2  3
2  9  8  7
3  4  5  6

In [138]: A.merge(B).drop_duplicates()
Out[138]:
   A  B  C
0  1  2  3
2  4  5  6

In [139]: len(A.merge(B).drop_duplicates()) == len(A.drop_duplicates())
Out[139]: True