Python Program to Count Lines in a File

Use with open('filename', 'r') as file: lines = sum(1 for line in file) to count lines in a file in Python.

Examples

Inputfile content: Hello\nWorld\n

Output2

Inputfile content: (empty file)

Output0

Inputfile content: Line1\nLine2\nLine3\nLine4\n

Output4

How to Think About It

To count lines in a file, open the file in read mode, then read each line one by one and keep a count. The total count after reading all lines is the number of lines in the file.

Algorithm

1

Open the file in read mode.

2

Initialize a counter to zero.

3

For each line in the file, increase the counter by one.

4

After reading all lines, close the file.

5

Return or print the counter value.

Code

python

filename = 'sample.txt'
with open(filename, 'r') as file:
    line_count = sum(1 for line in file)
print(f'Total lines: {line_count}')

Output

Total lines: 4

Dry Run

Let's trace counting lines in a file with 4 lines: 'Line1\nLine2\nLine3\nLine4\n'.

1

Open file

Open 'sample.txt' in read mode.

2

Count lines

Read each line and count: Line1 (count=1), Line2 (count=2), Line3 (count=3), Line4 (count=4).

3

Close file

File is automatically closed after the with block.

4

Print result

Print 'Total lines: 4'.

Line Content	Count After Reading
Line1	1
Line2	2
Line3	3
Line4	4

Why This Works

Step 1: Opening the file

Using with open(filename, 'r') opens the file safely and ensures it closes automatically.

Step 2: Counting lines

The expression sum(1 for line in file) reads each line and adds 1, counting total lines efficiently.

Step 3: Outputting the count

Printing the count shows how many lines the file contains.

Alternative Approaches

Using readlines()

python

filename = 'sample.txt'
with open(filename, 'r') as file:
    lines = file.readlines()
    line_count = len(lines)
print(f'Total lines: {line_count}')

Reads all lines into memory at once, which can be slower and use more memory for large files.

Using a loop and counter

python

filename = 'sample.txt'
line_count = 0
with open(filename, 'r') as file:
    for line in file:
        line_count += 1
print(f'Total lines: {line_count}')

Explicit loop is clear and memory efficient, good for beginners to understand counting.

Complexity: O(n) time, O(1) space

Time Complexity

The program reads each line once, so time grows linearly with the number of lines, O(n).

Space Complexity

Using a generator expression counts lines without extra memory, so space is O(1).

Which Approach is Fastest?

Using sum(1 for line in file) is efficient and memory-friendly compared to reading all lines at once.

Approach	Time	Space	Best For
Generator expression with sum	O(n)	O(1)	Large files, memory efficient
readlines() and len()	O(n)	O(n)	Small files, simple code
Explicit loop and counter	O(n)	O(1)	Clear logic, beginner-friendly