how do you filter pandas dataframes by multiple columns

To filter a dataframe (df) by a single column, if we consider data with male and females we might:

males = df[df[Gender]=='Male']

Question 1 - But what if the data spanned multiple years and i wanted to only see males for 2014?

In other languages I might do something like:

if A = "Male" and if B = "2014" then

(except I want to do this and get a subset of the original dataframe in a new dataframe object)

Question 2. How do I do this in a loop, and create a dataframe object for each unique sets of year and gender (i.e. a df for: 2013-Male, 2013-Female, 2014-Male, and 2014-Female

for y in year:

for g in gender:

df = .....

Using & operator, don't forget to wrap the sub-statements with ():

males = df[(df[Gender]=='Male') & (df[Year]==2014)]

To store your dataframes in a dict using a for loop:

from collections import defaultdict
dic={}
for g in ['male', 'female']:
  dic[g]=defaultdict(dict)
  for y in [2013, 2014]:
    dic[g][y]=df[(df[Gender]==g) & (df[Year]==y)] #store the DataFrames to a dict of dict

EDIT:

A demo for your getDF:

def getDF(dic, gender, year):
  return dic[gender][year]

print genDF(dic, 'male', 2014)

For more general boolean functions that you would like to use as a filter and that depend on more than one column, you can use:

df = df[df[['col_1','col_2']].apply(lambda x: f(*x), axis=1)]

where f is a function that is applied to every pair of elements (x1, x2) from col_1 and col_2 and returns True or False depending on any condition you want on (x1, x2).

Start from pandas 0.13, this is the most efficient way.

df.query('Gender=="Male" & Year=="2014" ')

In case somebody wonders what is the faster way to filter (the accepted answer or the one from @redreamality):

import pandas as pd
import numpy as np

length = 100_000
df = pd.DataFrame()
df['Year'] = np.random.randint(1950, 2019, size=length)
df['Gender'] = np.random.choice(['Male', 'Female'], length)

%timeit df.query('Gender=="Male" & Year=="2014" ')
%timeit df[(df['Gender']=='Male') & (df['Year']==2014)]

Results for 100,000 rows:

6.67 ms ± 557 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)
5.54 ms ± 536 µs per loop (mean ± std. dev. of 7 runs, 100 loops each)

Results for 10,000,000 rows:

326 ms ± 6.52 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)
472 ms ± 25.1 ms per loop (mean ± std. dev. of 7 runs, 1 loop each)

So results depend on the size and the data. On my laptop, query() gets faster after 500k rows. Further, the string search in Year=="2014" has an unnecessary overhead (Year==2014 is faster).

You can create your own filter function using query in pandas. Here you have filtering of df results by all the kwargs parameters. Dont' forgot to add some validators(kwargs filtering) to get filter function for your own df.

def filter(df, **kwargs):
    query_list = []
    for key in kwargs.keys():
        query_list.append(f'{key}=="{kwargs[key]}"')
    query = ' & '.join(query_list)
    return df.query(query)

how do you filter pandas dataframes by multiple columns

EDIT:

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