Reading CSV files (read_csv) in Data Analysis Python - Mini Project: Build & Apply

Reading CSV files (read_csv)

📖 Scenario: You work in a small bookstore. You have a CSV file listing books with their titles, authors, and prices. You want to read this file to see the data inside.

🎯 Goal: Learn how to use pandas.read_csv() to load a CSV file into a DataFrame and display its contents.

📋 What You'll Learn

Use the pandas library to read CSV files

Create a CSV file named books.csv with given data

Read the CSV file into a DataFrame called books_df

Print the DataFrame to see the data

💡 Why This Matters

🌍 Real World

Reading CSV files is a common task to load data from spreadsheets or exported reports into Python for analysis.

💼 Career

Data analysts and scientists often start by reading CSV files to explore and clean data before deeper analysis.

1

Create the CSV file books.csv

Create a CSV file named books.csv with these exact contents including the header line:
Title,Author,Price
The Alchemist,Paulo Coelho,10.99
1984,George Orwell,8.99
Python Basics,John Doe,15.50

Data Analysis Python

# Create the CSV file 'books.csv' with the exact content
# Your code here

Hint

Use Python's open function with mode 'w' to write text to a file.

2

Import pandas library

Write a line to import the pandas library using the alias pd.

Data Analysis Python

import pandas as pd

# Your code here

Hint

Use import pandas as pd to import the pandas library with the common alias.

3

Read the CSV file into a DataFrame

Use pd.read_csv() to read the file books.csv into a variable called books_df.

Data Analysis Python

import pandas as pd

csv_content = '''Title,Author,Price
The Alchemist,Paulo Coelho,10.99
1984,George Orwell,8.99
Python Basics,John Doe,15.50
'''

with open('books.csv', 'w') as file:
    file.write(csv_content)

# Use pd.read_csv() to read 'books.csv' into books_df
# Your code here

Hint

Use books_df = pd.read_csv('books.csv') to load the CSV file into a DataFrame.

4

Print the DataFrame books_df

Write a line to print the DataFrame books_df to see the data.

Data Analysis Python

import pandas as pd

csv_content = '''Title,Author,Price
The Alchemist,Paulo Coelho,10.99
1984,George Orwell,8.99
Python Basics,John Doe,15.50
'''

with open('books.csv', 'w') as file:
    file.write(csv_content)

books_df = pd.read_csv('books.csv')

# Print the DataFrame books_df
# Your code here

Hint

Use print(books_df) to display the DataFrame.

Practice

1. What does the read_csv function in pandas do?

easy

A. Saves a DataFrame to a CSV file

B. Loads data from a CSV file into a DataFrame

C. Deletes a CSV file from disk

D. Creates a new empty CSV file

Solution

1. Step 1: Understand the purpose of read_csv

   The read_csv function is used to load data from a CSV file into a pandas DataFrame, which is a table-like data structure.

2. Step 2: Differentiate from other file operations

   Saving or deleting files are done by other functions, not read_csv. This function only reads and loads data.

3. Final Answer:

   Loads data from a CSV file into a DataFrame -> Option B

4. Quick Check:

   read_csv = Load CSV data [OK]

Hint: Remember: read_csv means read and load CSV data [OK]

Common Mistakes:

• Confusing read_csv with saving functions
• Thinking it creates or deletes files
• Assuming it returns raw text instead of DataFrame

2. Which of the following is the correct syntax to load a CSV file named data.csv using pandas?

easy

A. pd.read_csv('data.csv')

B. pd.load_csv('data.csv')

C. pandas.read('data.csv')

D. pd.read('data.csv')

Solution

1. Step 1: Recall pandas function for reading CSV

   The correct pandas function to read CSV files is read_csv, called as pd.read_csv() if pandas is imported as pd.

2. Step 2: Check the syntax correctness

   The argument should be the filename as a string inside parentheses. So pd.read_csv('data.csv') is correct.

3. Final Answer:

   pd.read_csv('data.csv') -> Option A

4. Quick Check:

   Correct function name and syntax = pd.read_csv('data.csv') [OK]

Hint: Use pd.read_csv('filename.csv') to load CSV files [OK]

Common Mistakes:

• Using pd.load_csv which does not exist
• Using pd.read instead of read_csv
• Forgetting quotes around filename

3. Given the CSV file sample.csv with content:

name,age
Alice,30
Bob,25
Charlie,35

What will be the output of this code?

import pandas as pd
df = pd.read_csv('sample.csv')
print(df.head(2))

medium

A. Throws an error because head() needs no arguments

B. Prints all rows because head() defaults to all

C. Prints only the column names without data

D. Prints first 2 rows with columns 'name' and 'age'

Solution

1. Step 1: Understand what read_csv does here

   The code loads the CSV file into a DataFrame with columns 'name' and 'age' and three rows.

2. Step 2: Understand head(2) output

   The head(2) function returns the first 2 rows of the DataFrame, including column headers.

3. Final Answer:

   Prints first 2 rows with columns 'name' and 'age' -> Option D

4. Quick Check:

   head(2) = first 2 rows [OK]

Hint: head(n) shows first n rows of DataFrame [OK]

Common Mistakes:

• Thinking head() shows all rows
• Assuming head() requires no arguments
• Expecting only column names without data

4. What is wrong with this code snippet?

import pandas as pd
df = pd.read_csv(data.csv)
print(df.head())

medium

A. Missing quotes around filename 'data.csv'

B. Using print instead of display

C. read_csv cannot be used without specifying separator

D. head() cannot be called without an argument

Solution

1. Step 1: Check the argument passed to read_csv

   The filename must be a string, so it needs quotes around it. Here, data.csv is not quoted, causing a NameError.

2. Step 2: Verify other parts of the code

   Using print is fine, separator defaults to comma, and head() can be called without arguments.

3. Final Answer:

   Missing quotes around filename 'data.csv' -> Option A

4. Quick Check:

   Filename must be a string in quotes [OK]

Hint: Always put filename in quotes in read_csv [OK]

Common Mistakes:

• Forgetting quotes around filename
• Thinking head() needs an argument
• Assuming separator must always be specified

5. You have a CSV file records.csv where values are separated by semicolons (;). Which code correctly loads this file into a DataFrame?

hard

A. pd.read_csv('records.csv', sep=',')

B. pd.read_csv('records.csv', delimiter=',')

C. pd.read_csv('records.csv', sep=';')

D. pd.read_csv('records.csv', sep='|')

Solution

1. Step 1: Identify the separator used in the CSV file

   The file uses semicolons (;) to separate values, so the separator must be set to ';'.

2. Step 2: Use the correct parameter in read_csv

   The parameter sep=';' tells pandas to split columns on semicolons instead of commas.

3. Final Answer:

   pd.read_csv('records.csv', sep=';') -> Option C

4. Quick Check:

   Set sep to actual delimiter used [OK]

Hint: Use sep=';' if CSV uses semicolons as separators [OK]

Common Mistakes:

• Assuming comma is always the separator
• Using wrong sep parameter
• Confusing sep and delimiter parameters