pandas multiple conditions while indexing data frame - unexpected behavior

Question

I am filtering rows in a dataframe by values in two columns   For some reason the OR operator behaves like I would expect AND operator to behave and vice versa   My test code   import pandas as pd  df   pd DataFrame   a   range 5    b   range 5        let s insert some -1 values df  a   1    -1 df  b   1    -1 df  a   3    -1 df  b   4    -1  df1   df  df a    -1   amp   df b    -1   df2   df  df a    -1     df b    -1    print pd concat  df  df1  df2   axis 1                  keys      original df    using AND   amp      using OR           And the result         original df      using AND   amp        using OR                      a  b              a   b             a   b 0            0  0              0   0             0   0 1           -1 -1            NaN NaN           NaN NaN 2            2  2              2   2             2   2 3           -1  3            NaN NaN            -1   3 4            4 -1            NaN NaN             4  -1   5 rows x 6 columns    As you can see  the AND operator drops every row in which at least one value equals -1  On the other hand  the OR operator requires both values to be equal to -1 to drop them  I would expect exactly the opposite result  Could anyone explain this behavior  please   I am using pandas 0 13 1

User · Answer

A little mathematical logic theory here    NOT a AND NOT b  is the same as  NOT  a OR b    so    a NOT -1 AND b NOT -1   is equivalent of   NOT  a is -1 OR b is -1    which is opposite  Complement  of   a is -1 OR b is -1     So if you want exact opposite result  df1 and df2 should be as below   df1   df  df a    -1   amp   df b    -1   df2   df  df a    -1     df b    -1

User · Answer

You can also use query    i e   df filtered   df query  a    4  amp  b    2

User · Answer

As you can see  the AND operator drops every row in which at least one   value equals -1  On the other hand  the OR operator requires both   values to be equal to -1 to drop them    That s right   Remember that you re writing the condition in terms of what you want to keep  not in terms of what you want to drop   For df1   df1   df  df a    -1   amp   df b    -1     You re saying  keep the rows in which df a isn t -1 and df b isn t -1   which is the same as dropping every row in which at least one value is -1   For df2   df2   df  df a    -1     df b    -1     You re saying  keep the rows in which either df a or df b is not -1   which is the same as dropping rows where both values are -1   PS  chained access like df  a   1    -1 can get you into trouble   It s better to get into the habit of using  loc and  iloc

[python] pandas: multiple conditions while indexing data frame - unexpected behavior

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