Python tell Function
Last modified March 26, 2025
This comprehensive guide explores Python's tell function, which
returns the current position of the file pointer. We'll cover its usage with
different file modes, practical examples, and best practices for file position
tracking in Python.
Basic Definition
The tell() method returns an integer representing the current
position of the file pointer in the file. This position is measured in bytes
from the beginning of the file. It's useful for tracking progress or returning
to specific positions later.
When a file is opened, the position starts at 0 (beginning of file). Reading
or writing operations move the pointer forward. The tell function
works with both text and binary files, though behavior differs slightly.
Basic tell Usage
This example demonstrates the simplest use of tell to show the
current file position before and after reading some data.
with open('example.txt', 'r') as file:
print("Initial position:", file.tell())
data = file.read(10) # Read first 10 bytes
print("After reading 10 bytes:", file.tell())
print("Data read:", data)
The code opens a file and immediately checks the position (0). After reading
10 bytes, the position advances to 10. This shows how tell tracks
the file pointer's movement during operations.
tell with Binary Files
Binary files provide precise position tracking since no character encoding conversion occurs. This example shows byte-accurate positioning.
with open('image.jpg', 'rb') as file:
print("Start:", file.tell())
file.read(100) # Skip header
print("After header:", file.tell())
chunk = file.read(1024)
print("After 1KB read:", file.tell())
This reads a binary file (JPEG image), showing positions after skipping a
100-byte header and reading a 1KB chunk. Binary mode ensures each byte read
corresponds exactly to the position reported by tell.
tell with Text Files
Text files may show different behavior due to newline conversions and encoding. This example demonstrates position tracking in text mode.
with open('textfile.txt', 'r') as file:
print("Position before read:", file.tell())
line = file.readline()
print("First line:", line)
print("Position after read:", file.tell())
file.seek(0)
print("Position after seek:", file.tell())
The code reads a line and shows positions before and after. Note that in text
mode, tell may not correspond exactly to byte positions due to
newline translations. The position is still valid for use with seek.
tell in Write Mode
When writing files, tell shows where the next write will occur.
This example demonstrates position tracking during file writing.
with open('output.txt', 'w') as file:
print("Start position:", file.tell())
file.write("First line\n")
print("After first write:", file.tell())
file.write("Second line\n")
print("After second write:", file.tell())
Each write operation advances the file pointer, shown by tell. In
write mode, the position always indicates where new data will be written. The
\n characters count as one byte each in the position.
tell with seek Combination
This example shows how tell and seek work together
for random file access.
with open('data.bin', 'rb+') as file:
print("Initial position:", file.tell())
file.seek(50)
print("After seek to 50:", file.tell())
file.write(b'X')
print("After write:", file.tell())
file.seek(0, 2) # Seek to end
print("File size:", file.tell())
The code demonstrates moving to position 50, writing a byte, then checking the
file size by seeking to the end. The tell function verifies each
operation's effect on the file position.
Best Practices
- Use with binary files: For precise byte positioning
- Combine with seek: For random access patterns
- Watch text mode behavior: Positions may not match byte counts
- Check before operations: Verify positions when working with fixed-size records
Source References
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