[Check if value in df col] You have to check if `val in col.values`, not just `val in col` #pandas #dangerous

val_in_df_col.py

# Doing `x in df["col"]` will always look for x in the *index* of the df, not the actual column values
# Thus, you have to use `x in df["col"].values` instead
# This can be especially insidious when you're checking if a number is in a col, but that number is also in the index.
#   If you don't use `.values` in this case, you will think the value is in the col when it actually isn't!

>>> df = pd.DataFrame({"a": [20, 40, 1], "b": [32, 42, 52]})
>>> df
    a   b
0  20  32
1  40  42
2   1  52
>>> 1 in df["a"]
True
>>> 20 in df["a"]
False
>>> 1 in df["a"].values
True
>>> 20 in df["a"].values
True