Pandas Replace is giving me a strange error

Question

Pandas is giving a weird output when using a dictionary to replace values within a dataframe:

import pandas as pd

df = pd.read_csv('data.csv')
print(df)
Course
English 21st Century
Maths in the Golden Age of History
Science is cool


Mapped_Items = ['Math', 'English', 'Science', 'History']

pat = '|'.join(r"\b{}\b".format(x) for x in Mapped_Items)
df['Interest'] = df['Course].str.findall('('+ pat + ')').str.join(', ')

mapped_dict = {'English' : 'Eng', 'Science' : 'Sci', 'Math' : 'Mat', 'History' : 'Hist'}
df['Interest'] = df1['Interest'].replace(mapped_dict, inplace=False)

What I get:

print(df)
df
Course                                Interest
English 21st Century                  Engg
Maths in the Golden Age of History    MatttHistt
Science is cool                       Scii

What I'm after is something close to the following :

 Course                               Interests
English 21st Century                  Eng
Maths in the Golden Age of History    Mat, Hist
Science is cool                       Sci

Answer 1

Your logic seems overcomplicated. You don't need regex, and pd.Series.replace is inefficient with a dictionary, even if it could work on a series of lists. Here's an alternative method:

import pandas as pd
from io import StringIO

mystr = StringIO("""Course
English 21st Century
Maths in the Golden Age of History
Science is cool""")

df = pd.read_csv(mystr)

d = {'English' : 'Eng', 'Science' : 'Sci', 'Math' : 'Mat', 'History' : 'Hist'}

df['Interest'] = df['Course'].apply(lambda x: ', '.join([d[i] for i in d if i in x]))

print(df)

                               Course   Interest
0                English 21st Century        Eng
1  Maths in the Golden Age of History  Mat, Hist
2                     Science is cool        Sci