Opening Files for Reading: Mode 'r'

A File with Yesterday's Notes

Imagine that notes.txt already contains important data from yesterday. You want a Python program to save today's notes, so you open the file using write mode, written as 'w'. The critical question is what happens before the program writes today's notes. Write mode does not preserve the old contents: opening the file immediately erases them.

The Instant State Change

The mode argument determines how Python works with a file. In 'w' mode, Python opens the file for writing and truncates it. Truncation means erasing the file's existing content. This happens at the instant the file is opened, before the program supplies any replacement text. If the file does not already exist, Python creates it instead.

file = open('notes.txt', 'w')

After the open call completes, file refers to a file object that represents notes.txt opened in write mode. The important file-state change has already happened: if notes.txt existed, its previous contents are gone. The variable named file does not contain the old text; it represents the opened file that is now ready for writing.

Opening Before Writing

The order matters. First, Python opens the file in 'w' mode. Next, it truncates the file immediately. Only after that can the program write new content. Therefore, writing new data does not add to the old contents: it writes into a file whose previous contents have already been removed.

file = open('report.txt', 'w')
file.write('Fresh report')

Fresh report

Write Mode and Append Mode

Choose 'w' when the intended result is a fresh replacement of the file's contents. Choose 'a' when the existing contents must remain and new data should be added at the end. Both modes can be used for writing, but they have opposite effects on data that is already present.

Mistakes with Existing Data

• Assuming 'w' adds new text after the old text.

  Write mode truncates the file as soon as it is opened, so Old notes are erased before Today's notes is written.

  Fix: Use 'a' when the existing text must be preserved and new text must be added at the end.

• Thinking truncation happens only when the write operation runs.

  Truncation happens at the instant the file is opened, even if no new data is subsequently written.

  Fix: Predict the file's state immediately after the open call: an existing file is already empty.

• Treating 'w' as unsafe in every situation.

  Replacing a file is exactly the intended behavior when old contents are no longer needed.

  Fix: Use 'w' deliberately when the entire file should be replaced, and verify the filename first.

Predict Before You Run

The prediction is an empty file immediately after the open call. If yesterday's result must remain, use 'a' rather than 'w', because append mode preserves existing content and adds new data at the end.

Key Takeaways

1. The 'w' mode opens a file for writing.
2. Opening an existing file in 'w' mode immediately erases its previous contents.
3. Truncation occurs before any new data is written.
4. If the file does not exist, Python creates it in 'w' mode.
5. Use 'w' to replace contents and 'a' to preserve existing contents while adding data at the end.