15 Python Snippets to Optimize your Data Science Pipeline

Quick Python solutions to help your data science cycle

Why Snippets Matter for Data Science

In my daily routine I have to deal with a lot of the same situations from loading csv files to visualizing data. So, to help streamline my process I created the habit of storing snippets of code that are helpful in different situations from loading csv files to visualizing data.

In this post I will share 15 snippets of code to help with different aspects of your data analysis pipeline

1. Loading multiple files with glob and list comprehension

import glob
import pandas as pd
csv_files = glob.glob("path/to/folder/with/csvs/*.csv")
dfs = [pd.read_csv(filename) for filename in csv_files]

2. Getting unique values from a column table

import pandas as pd
df = pd.read_csv("path/to/csv/file.csv")
df["Item_Identifier"].unique()

array(['FDA15', 'DRC01', 'FDN15', ..., 'NCF55', 'NCW30', 'NCW05'],
      dtype=object)

3. Display pandas dataframes side by side

from IPython.display import display_html
from itertools import chain,cycle

def display_side_by_side(*args,titles=cycle([''])):
    # source: https://stackoverflow.com/questions/38783027/jupyter-notebook-display-two-pandas-tables-side-by-side
    html_str=''
    for df,title in zip(args, chain(titles,cycle(['</br>'])) ):
        html_str+='<th style="text-align:center"><td style="vertical-align:top">'
        html_str+="<br>"
        html_str+=f'<h2>{title}</h2>'
        html_str+=df.to_html().replace('table','table style="display:inline"')
        html_str+='</td></th>'
    display_html(html_str,raw=True)
df1 = pd.read_csv("file.csv")
df2 = pd.read_csv("file2")
display_side_by_side(df1.head(),df2.head(), titles=['Sales','Advertising'])
### Output

image by the author

4. Remove all NaNs in pandas dataframe

df = pd.DataFrame(dict(a=[1,2,3,None]))
df
df.dropna(inplace=True)
df

5. Show number of NaN entries in DataFrame columns

def findNaNCols(df):
    for col in df:
        print(f"Column: {col}")
        num_NaNs = df[col].isnull().sum()
        print(f"Number of NaNs: {num_NaNs}")
df = pd.DataFrame(dict(a=[1,2,3,None],b=[None,None,5,6]))
findNaNCols(df)

# Output

Column: a
Number of NaNs: 1
Column: b
Number of NaNs: 2

6. Transforming columns with .apply and lambda functions

df = pd.DataFrame(dict(a=[10,20,30,40,50]))
square = lambda x: x**2
df["a"]=df["a"].apply(square)
df

7. Transforming 2 DataFrame columns into a dictionary

df = pd.DataFrame(dict(a=["a","b","c"],b=[1,2,3]))
df_dictionary = dict(zip(df["a"],df["b"]))
df_dictionary

{'a': 1, 'b': 2, 'c': 3}

8. Plotting grid of distributions with conditionals on columns

import numpy as np
import matplotlib.pyplot as plt
import seaborn as sns
sns.set()
import pandas as pd

df = pd.DataFrame(dict(a=np.random.randint(0,100,100),b=np.arange(0,100,1)))
plt.figure(figsize=(15,7))
plt.subplot(1,2,1)
df["b"][df["a"]>50].hist(color="green",label="bigger than 50")
plt.legend()
plt.subplot(1,2,2)
df["b"][df["a"]<50].hist(color="orange",label="smaller than 50")
plt.legend()
plt.show()

image by the author

9. Running t-tests for values of different columns in pandas

from scipy.stats import ttest_rel

data = np.arange(0,1000,1)
data_plus_noise = np.arange(0,1000,1) + np.random.normal(0,1,1000)
df = pd.DataFrame(dict(data=data, data_plus_noise=data_plus_noise))
print(ttest_rel(df["data"],df["data_plus_noise"]))

# Output
Ttest_relResult(statistic=-1.2717454718006775, pvalue=0.20375954602300195)

10. Merging dataframes on a given column

df1 = pd.DataFrame(dict(a=[1,2,3],b=[10,20,30],col_to_merge=["a","b","c"]))
df2 = pd.DataFrame(dict(d=[10,20,100],col_to_merge=["a","b","c"]))
df_merged = df1.merge(df2, on='col_to_merge')
df_merged

11. Normalizing values in a pandas column with sklearn

from sklearn.preprocessing import MinMaxScaler
scaler = MinMaxScaler()
scores = scaler.fit_transform(df["a"].values.reshape(-1,1))

12. Dropping NaNs in a specific column in pandas

df.dropna(subset=["col_to_remove_NaNs_from"],inplace=True)

13. Selecting subset of a dataframe with conditionals and or statement

df = pd.DataFrame(dict(result=["Pass","Fail","Pass","Fail","Distinction","Distinction"]))
pass_index = (df["result"]=="Pass") | (df["result"]=="Distinction")
df_pass = df[pass_index]
df_pass

14. Basic pie chart

import matplotlib.pyplot as plt

df = pd.DataFrame(dict(a=[10,20,50,10,10],b=["A","B","C","D","E"]))
labels = df["b"]
sizes = df["a"]
plt.pie(sizes, labels=labels, autopct='%1.1f%%', shadow=True, startangle=140)
plt.axis('equal')
plt.show()

15. Changing a percentage string to a numerical value using .apply()

def change_to_numerical(x):
    try:
        x = int(x.strip("%")[:2])
    except:
        x = int(x.strip("%")[:1])
    return x

df = pd.DataFrame(dict(a=["A","B","C"],col_with_percentage=["10%","70%","20%"]))
df["col_with_percentage"] = df["col_with_percentage"].apply(change_to_numerical)
df

Conclusion

I think snippets of code are super valuable, rewriting code can be a real waste of time so having a complete toolkit with all the simple solutions you need to streamline your data analysis process can be of great help.

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Further reading: Pandas Cheatsheet.