When creating a boolean series from subsetting a df, what does index-subsetting in the same line fil-CodePudding

I'm experimenting with pandas and noticed something that seemed odd.

If you have a boolean series defined as an object, you can then subset that object by index numbers, e.g.,

From df 'ah' , creating creating this boolean series, 'tah' via

tah = ah['_merge']=='left_only'

This boolean series could be index-subset like this:

tah[0:1]

yielding:

Yet if I tried to do this all in one line

ah['_merge']=='left_only'[0:1]

I get an unexpected output, where the boolean series is neither sliced nor seems to correspond to the subsetted-column:

I've been experimenting and can't seem to determine what, in the all-in-on-line [0:1] is slicing/filtering-for. Any clarification would be appreciated!

CodePudding user response：

Because you are equating string's 'left_over' index of 0 (letter 'l') and this yields false result in every row and since 'l' is not equal to 'left_over' nor to 'both', it prints all a column of false booleans.

CodePudding user response：

You can use (ah['_merge']=='left_only')[0:1] as MattDMo mentionned in the comments.

Or you can also use pandas.Series.iloc with a slice object to select the elements you need based on their postion/index in your dataframe.

(ah['_merge']=='left_only').iloc[0:1]

Both of commands will return True since the first row of your dataframe has a 'left_only' type of merge.