Reading files line by line in Python - Time & Space Complexity

Time Complexity: Reading files line by line

O(n)

Understanding Time Complexity

When reading a file line by line, we want to know how the time it takes grows as the file gets bigger.

We ask: How does the program's work increase when there are more lines to read?

Scenario Under Consideration

Analyze the time complexity of the following code snippet.

with open('file.txt', 'r') as file:
    for line in file:
        print(line.strip())

This code opens a file and reads it one line at a time, printing each line after removing extra spaces.

Identify Repeating Operations

Identify the loops, recursion, array traversals that repeat.

• Primary operation: Looping through each line in the file.
• How many times: Once for every line in the file.

How Execution Grows With Input

As the number of lines grows, the program reads and processes more lines one by one.

Input Size (n)	Approx. Operations
10	About 10 line reads and prints
100	About 100 line reads and prints
1000	About 1000 line reads and prints

Pattern observation: The work grows directly with the number of lines; double the lines, double the work.

Final Time Complexity

Time Complexity: O(n)

This means the time to read the file grows in a straight line with the number of lines.

Common Mistake

[X] Wrong: "Reading a file line by line is always very slow because it reads each character individually."

[OK] Correct: The code reads line by line, not character by character, so it processes one line at a time efficiently.

Interview Connect

Understanding how reading files scales helps you write programs that handle big data smoothly and shows you think about efficiency.

Self-Check

"What if we read the whole file at once into memory instead of line by line? How would the time complexity change?"