set difference for pandas
PythonPandasDataframePython Problem Overview
A simple pandas question:
Is there a drop_duplicates() functionality to drop every row involved in the duplication?
An equivalent question is the following: Does pandas have a set difference for dataframes?
For example:
In [5]: df1 = pd.DataFrame({'col1':[1,2,3], 'col2':[2,3,4]})
In [6]: df2 = pd.DataFrame({'col1':[4,2,5], 'col2':[6,3,5]})
In [7]: df1
Out[7]:
col1 col2
0 1 2
1 2 3
2 3 4
In [8]: df2
Out[8]:
col1 col2
0 4 6
1 2 3
2 5 5
so maybe something like df2.set_diff(df1) will produce this:
col1 col2
0 4 6
2 5 5
However, I don't want to rely on indexes because in my case, I have to deal with dataframes that have distinct indexes.
By the way, I initially thought about an extension of the current drop_duplicates() method, but now I realize that the second approach using properties of set theory would be far more useful in general. Both approaches solve my current problem, though.
Thanks!
Python Solutions
Solution 1 - Python
Bit convoluted but if you want to totally ignore the index data. Convert the contents of the dataframes to sets of tuples containing the columns:
ds1 = set(map(tuple, df1.values))
ds2 = set(map(tuple, df2.values))
This step will get rid of any duplicates in the dataframes as well (index ignored)
set([(1, 2), (3, 4), (2, 3)]) # ds1
can then use set methods to find anything. Eg to find differences:
ds1.difference(ds2)
gives: set([(1, 2), (3, 4)])
can take that back to dataframe if needed. Note have to transform set to list 1st as set cannot be used to construct dataframe:
pd.DataFrame(list(ds1.difference(ds2)))
Solution 2 - Python
Here's another answer that keeps the index and does not require identical indexes in two data frames. (EDIT: make sure there is no duplicates in df2 beforehand)
pd.concat([df2, df1, df1]).drop_duplicates(keep=False)
It is fast and the result is
col1 col2
0 4 6
2 5 5
Solution 3 - Python
from pandas import DataFrame
df1 = DataFrame({'col1':[1,2,3], 'col2':[2,3,4]})
df2 = DataFrame({'col1':[4,2,5], 'col2':[6,3,5]})
print(df2[~df2.isin(df1).all(1)])
print(df2[(df2!=df1)].dropna(how='all'))
print(df2[~(df2==df1)].dropna(how='all'))
Solution 4 - Python
Apply by the columns of the object you want to map (df2); find the rows that are not in the set (isin is like a set operator)
In [32]: df2.apply(lambda x: df2.loc[~x.isin(df1[x.name]),x.name])
Out[32]:
col1 col2
0 4 6
2 5 5
Same thing, but include all values in df1, but still per column in df2
In [33]: df2.apply(lambda x: df2.loc[~x.isin(df1.values.ravel()),x.name])
Out[33]:
col1 col2
0 NaN 6
2 5 5
2nd example
In [34]: g = pd.DataFrame({'x': [1.2,1.5,1.3], 'y': [4,4,4]})
In [35]: g.columns=df1.columns
In [36]: g
Out[36]:
col1 col2
0 1.2 4
1 1.5 4
2 1.3 4
In [32]: g.apply(lambda x: g.loc[~x.isin(df1[x.name]),x.name])
Out[32]:
col1 col2
0 1.2 NaN
1 1.5 NaN
2 1.3 NaN
Note, in 0.13, there will be an isin operator on the frame level, so something like: df2.isin(df1) should be possible
Solution 5 - Python
There are 3 methods which work, but two of them have some flaws.
Method 1 (Hash method):
It worked for all cases I tested.
df1.loc[:, "hash"] = df1.apply(lambda x: hash(tuple(x)), axis = 1)
df2.loc[:, "hash"] = df2.apply(lambda x: hash(tuple(x)), axis = 1)
df1 = df1.loc[~df1["hash"].isin(df2["hash"]), :]
Method 2 (Dict method):
It fails if DataFrames contain datetime columns.
df1 = df1.loc[~df1.isin(df2.to_dict(orient="list")).all(axis=1), :]
Method 3 (MultiIndex method):
I encountered cases when it failed on columns with None's or NaN's.
df1 = df1.loc[~df1.set_index(list(df1.columns)).index.isin(df2.set_index(list(df2.columns)).index)
Solution 6 - Python
Get the indices of the intersection with a merge, then drop them:
>>> df_all = pd.DataFrame(np.arange(8).reshape((4,2)), columns=['A','B']); df_all
A B
0 0 1
1 2 3
2 4 5
3 6 7
>>> df_completed = df_all.iloc[::2]; df_completed
A B
0 0 1
2 4 5
>>> merged = pd.merge(df_all.reset_index(), df_completed); merged
index A B
0 0 0 1
1 2 4 5
>>> df_pending = df_all.drop(merged['index']); df_pending
A B
1 2 3
3 6 7
Solution 7 - Python
Numpy's setdiff1d would work and perhaps be faster.
For each column:
np.setdiff1(df1.col1.values, df2.col1.values)
So something like:
setdf = pd.DataFrame({
col: np.setdiff1d(getattr(df1, col).values, getattr(df2, col).values)
for col in df1.columns
})
Solution 8 - Python
Assumption: > 1. df1 and df2 have identical columns > 2. it is a set operation so duplicates are ignored > 3. sets are not extremely large so you do not worry about memory
union = pd.concat([df1,df2])
sym_diff = union[~union.duplicated(keep=False)]
union_of_df1_and_sym_diff = pd.concat([df1, sym_diff])
diff = union_of_df1_and_sym_diff[union_of_df1_and_sym_diff.duplicated()]
Solution 9 - Python
Edit: You can now make MultiIndex objects directly from data frames as of pandas 0.24.0 which greatly simplifies the syntax of this answer
df1mi = pd.MultiIndex.from_frame(df1)
df2mi = pd.MultiIndex.from_frame(df2)
dfdiff = df2mi.difference(df1mi).to_frame().reset_index(drop=True)
Original Answer
Pandas MultiIndex objects have fast set operations implemented as methods, so you can convert the DataFrames to MultiIndexes, use the difference() method, then convert the result back to a DataFrame. This solution should be much faster (by ~100x or more from my brief testing) than the solutions given here so far, and it will not depend on the row indexing of the original frames. As Piotr mentioned for his answer, this will fail with null values, since np.nan != np.nan. Any row in df2 with a null value will always appear in the difference. Also, the columns should be in the same order for both DataFrames.
df1mi = pd.MultiIndex.from_arrays(df1.values.transpose(), names=df1.columns)
df2mi = pd.MultiIndex.from_arrays(df2.values.transpose(), names=df2.columns)
dfdiff = df2mi.difference(df1mi).to_frame().reset_index(drop=True)
Solution 10 - Python
I'm not sure how pd.concat() implicitly joins overlapping columns but I had to do a little tweak on @radream's answer.
Conceptually, a set difference (symmetric) on multiple columns is a set union (outer join) minus a set intersection (or inner join):
df1 = pd.DataFrame({'col1':[1,2,3], 'col2':[2,3,4]})
df2 = pd.DataFrame({'col1':[4,2,5], 'col2':[6,3,5]})
o = pd.merge(df1, df2, how='outer')
i = pd.merge(df1, df2)
set_diff = pd.concat([o, i]).drop_duplicates(keep=False)
This yields:
col1 col2
0 1 2
2 3 4
3 4 6
4 5 5
Solution 11 - Python
In Pandas 1.1.0 you can count unique rows with value_counts and find difference between counts:
df1 = pd.DataFrame({'col1':[1,2,3], 'col2':[2,3,4]})
df2 = pd.DataFrame({'col1':[4,2,5], 'col2':[6,3,5]})
diff = df2.value_counts().sub(df1.value_counts(), fill_value=0)
Result:
col1 col2
1 2 -1.0
2 3 0.0
3 4 -1.0
4 6 1.0
5 5 1.0
dtype: float64
Get positive counts:
diff[diff > 0].reset_index(name='counts')
col1 col2 counts
0 4 6 1.0
1 5 5 1.0
Solution 12 - Python
this should work even if you have multiple columns in both dataframes. But make sure that the column names of both the dataframes are the exact same.
set_difference = pd.concat([df2, df1, df1]).drop_duplicates(keep=False)
With multiple columns you can also use:
col_names=['col_1','col_2']
set_difference = pd.concat([df2[col_names], df1[col_names],
df1[col_names]]).drop_duplicates(keep=False)