Reading HTML tables in Data Analysis Python - Time & Space Complexity

Time Complexity: Reading HTML tables

O(n * m)

Understanding Time Complexity

When we read HTML tables into data, we want to know how long it takes as the table size grows.

We ask: How does the time to read change when the table has more rows or columns?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

import pandas as pd

url = 'https://example.com/sample-table.html'
tables = pd.read_html(url)
df = tables[0]

This code reads all HTML tables from a webpage and selects the first table as a DataFrame.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Parsing each row and column of the HTML table to build the DataFrame.
• How many times: Once for each cell in the table (rows x columns).

How Execution Grows With Input

As the number of rows and columns grows, the work grows with the total number of cells.

Input Size (rows x columns)	Approx. Operations
10 x 5 = 50	About 50 operations
100 x 10 = 1,000	About 1,000 operations
1,000 x 20 = 20,000	About 20,000 operations

Pattern observation: The time grows roughly in direct proportion to the number of cells in the table.

Final Time Complexity

Time Complexity: O(n * m)

This means the time to read the table grows proportionally to the number of rows (n) times the number of columns (m).

Common Mistake

[X] Wrong: "Reading an HTML table takes the same time no matter how big it is."

[OK] Correct: The code must process each cell, so bigger tables take more time to read.

Interview Connect

Understanding how data reading time grows helps you explain your code's efficiency clearly and confidently.

Self-Check

"What if the HTML table has nested tables inside cells? How would the time complexity change?"

Practice

1. What does the pd.read_html() function in pandas do?

easy

A. It converts a DataFrame into an HTML table string.

B. It reads all HTML tables from a webpage and returns them as a list of DataFrames.

C. It downloads an HTML file from the internet.

D. It parses JSON data from a webpage.

Solution

1. Step 1: Understand the purpose of pd.read_html()

   This function is designed to extract tables from HTML content on web pages.

2. Step 2: Recognize the output type

   The function returns a list where each element is a DataFrame representing one HTML table found.

3. Final Answer:

   It reads all HTML tables from a webpage and returns them as a list of DataFrames. -> Option B

4. Quick Check:

   pd.read_html() = list of DataFrames [OK]

Hint: Remember: read_html returns a list of tables, not a single table [OK]

Common Mistakes:

• Thinking it returns a single DataFrame instead of a list
• Confusing it with functions that write HTML
• Assuming it downloads files instead of parsing HTML

2. Which of the following is the correct way to read the first HTML table from a webpage URL stored in url using pandas?

easy

A. df = pd.read_html(url)[1]

B. df = pd.read_html(url, index=0)

C. df = pd.read_html(url).first()

D. tables = pd.read_html(url); df = tables[0]

Solution

1. Step 1: Recall the return type of pd.read_html()

   The function returns a list of DataFrames representing all tables found.

2. Step 2: Access the first table correctly

   Use list indexing [0] to get the first table from the list.

3. Final Answer:

   tables = pd.read_html(url); df = tables[0] -> Option D

4. Quick Check:

   Use list index 0 to get first table [OK]

Hint: Use index 0 on the list returned by read_html to get first table [OK]

Common Mistakes:

• Using .first() method which does not exist
• Passing index=0 as a parameter to read_html
• Using index 1 instead of 0 for first table

3. Given the code below, what will be the output of print(len(tables)) if the webpage contains 3 HTML tables?

import pandas as pd
url = 'https://example.com'
tables = pd.read_html(url)
print(len(tables))

medium

A. 0

B. 1

C. 3

D. Error: pd.read_html() requires a file path

Solution

1. Step 1: Understand what pd.read_html() returns

   It returns a list of DataFrames, one for each HTML table found on the page.

2. Step 2: Count the number of tables

   Since the webpage has 3 tables, the list length will be 3.

3. Final Answer:

   3 -> Option C

4. Quick Check:

   len(tables) = number of tables found [OK]

Hint: Count tables by checking length of list returned by read_html [OK]

Common Mistakes:

• Assuming it returns a single DataFrame
• Thinking it returns 0 if tables exist
• Believing it needs a local file path only

4. What is wrong with this code snippet to read the first HTML table from a URL?

import pandas as pd
url = 'https://example.com'
df = pd.read_html(url)[1]

medium

A. It tries to access the second table but may cause IndexError if only one table exists.

B. The syntax for pd.read_html is incorrect; it needs a file path, not a URL.

C. pd.read_html returns a DataFrame, so indexing with [1] is invalid.

D. The variable name df is reserved and cannot be used.

Solution

1. Step 1: Check the indexing used

   The code uses index [1], which means it tries to get the second table.

2. Step 2: Consider possible errors

   If the webpage has only one table, index 1 is out of range and causes IndexError.

3. Final Answer:

   It tries to access the second table but may cause IndexError if only one table exists. -> Option A

4. Quick Check:

   Indexing beyond list length causes error [OK]

Hint: Always check list length before accessing index [OK]

Common Mistakes:

• Assuming pd.read_html returns a single DataFrame
• Ignoring possible IndexError from wrong index
• Thinking URLs are invalid input for read_html

5. You want to extract the second table from a webpage but only if it contains a column named 'Price'. Which code snippet correctly reads the tables and selects the second one only if it has the 'Price' column?

hard

A. tables = pd.read_html(url)
if 'Price' in tables[1].columns:
df = tables[1]
else:
df = None

B. df = pd.read_html(url)[1]
if 'Price' in df.index:
pass
else:
df = None

C. tables = pd.read_html(url)
df = tables[1].filter('Price')

D. df = pd.read_html(url, match='Price')[1]

Solution

1. Step 1: Read all tables from the webpage

   Use pd.read_html(url) to get a list of tables.

2. Step 2: Check if the second table has the 'Price' column

   Access tables[1] and check if 'Price' is in its columns.

3. Step 3: Assign the table or None accordingly

   If the column exists, assign the table to df, else assign None.

4. Final Answer:

   tables = pd.read_html(url) if 'Price' in tables[1].columns: df = tables[1] else: df = None -> Option A

5. Quick Check:

   Check columns before selecting table [OK]

Hint: Check columns attribute before using a table [OK]

Common Mistakes:

• Using filter() incorrectly on DataFrame
• Assuming match parameter filters tables by column
• Not checking if index 1 exists before access