How to map numeric data into categories / bins in Pandas dataframe

Question

I've just started coding in python, and my general coding skills are fairly rusty :( so please be a bit patient

I have a pandas dataframe:

It has around 3m rows. There are 3 kinds of age_units: Y, D, W for years, Days & Weeks. Any individual over 1 year old has an age unit of Y and my first grouping I want is <2y old so all I have to test for in Age Units is Y...

I want to create a new column AgeRange and populate with the following ranges:

• <2
• 2 - 18
• 18 - 35
• 35 - 65
• 65+

so I wrote a function

def agerange(values):
    for i in values:
        if complete.Age_units == 'Y':
            if complete.Age > 1 AND < 18 return '2-18'
            elif complete.Age > 17 AND < 35 return '18-35'
            elif complete.Age > 34 AND < 65 return '35-65'
            elif complete.Age > 64 return '65+'
        else return '< 2'

I thought if I passed in the dataframe as a whole I would get back what I needed and then could create the column I wanted something like this:

agedetails['age_range'] = ageRange(agedetails)

BUT when I try to run the first code to create the function I get:

  File "<ipython-input-124-cf39c7ce66d9>", line 4
    if complete.Age > 1 AND complete.Age < 18 return '2-18'
                          ^
SyntaxError: invalid syntax

Clearly it is not accepting the AND - but I thought I heard in class I could use AND like this? I must be mistaken but then what would be the right way to do this?

So after getting that error, I'm not even sure the method of passing in a dataframe will throw an error either. I am guessing probably yes. In which case - how would I make that work as well?

I am looking to learn the best method, but part of the best method for me is keeping it simple even if that means doing things in a couple of steps...

Solution

With Pandas, you should avoid row-wise operations, as these usually involve an inefficient Python-level loop. Here are a couple of alternatives.

Pandas: pd.cut

As @JonClements suggests, you can use pd.cut for this, the benefit here being that your new column becomes a Categorical.

You only need to define your boundaries (including np.inf) and category names, then apply pd.cut to the desired numeric column.

bins = [0, 2, 18, 35, 65, np.inf]
names = ['<2', '2-18', '18-35', '35-65', '65+']

df['AgeRange'] = pd.cut(df['Age'], bins, labels=names)

print(df.dtypes)

# Age             int64
# Age_units      object
# AgeRange     category
# dtype: object

NumPy: np.digitize

np.digitize provides another clean solution. The idea is to define your boundaries and names, create a dictionary, then apply np.digitize to your Age column. Finally, use your dictionary to map your category names.

Note that for boundary cases the lower bound is used for mapping to a bin.

import pandas as pd, numpy as np

df = pd.DataFrame({'Age': [99, 53, 71, 84, 84],
                   'Age_units': ['Y', 'Y', 'Y', 'Y', 'Y']})

bins = [0, 2, 18, 35, 65]
names = ['<2', '2-18', '18-35', '35-65', '65+']

d = dict(enumerate(names, 1))

df['AgeRange'] = np.vectorize(d.get)(np.digitize(df['Age'], bins))

Result

   Age Age_units AgeRange
0   99         Y      65+
1   53         Y    35-65
2   71         Y      65+
3   84         Y      65+
4   84         Y      65+