In [1]: df
Out[1]:
data
0 1
1 2
2 3
3 4
You want to apply a function that conditionally returns a value based on the selected dataframe column.
In [2]: df['data'].apply(lambda x: 'true' if x <= 2.5 else 'false')
Out[2]:
0 true
1 true
2 false
3 false
Name: data
You can then assign that returned column to a new column in your dataframe:
In [3]: df['desired_output'] = df['data'].apply(lambda x: 'true' if x <= 2.5 else 'false')
In [4]: df
Out[4]:
data desired_output
0 1 true
1 2 true
2 3 false
3 4 false