To clear an existing file before writing new text, open it in write mode: with open("example.txt", "w", encoding="utf-8") as f:. Opening with "w" truncates the file immediately; it also creates the file if it does not exist and your process has permission to do so.
Choose the right way to clear the file
| Situation | Use | What it does |
|---|---|---|
| Clear a file before writing text | open(path, "w", encoding="utf-8") |
Truncates the file as it opens; creates it if missing, subject to filesystem permissions. |
| Clear a file before writing binary data | open(path, "wb") |
Truncates the file and works with bytes rather than text. |
| The file is already open and should stay open | f.seek(0), then f.truncate(0) |
Resizes the existing writable stream to zero bytes. |
| Operate on a path or file descriptor | os.truncate(path, 0) or os.ftruncate(fd, 0) |
Resizes the file through the operating-system interface. |
Clear a file by opening it in write mode
Use this when you want to discard the old contents and then write new text. The file is cleared on opening, before any later write happens.
with open("example.txt", "w", encoding="utf-8") as f:
f.write("New contentsn")For predictable text encoding, specify one such as UTF-8; otherwise, the default text encoding depends on the platform. Text mode reads and writes str. For binary data, use "wb" and write bytes instead. See the Python documentation for open() modes and the text and binary I/O tutorial.
Use "w+" if you need to read from the newly opened file as well as write to it. Do not choose "r+" expecting it to clear the file: that mode opens an existing file for updating but does not truncate it.
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Clear an already-open file object
If you need to keep using an open, writable, seekable file object, move to the beginning and truncate there:
with open("example.txt", "r+", encoding="utf-8") as f:
f.seek(0)
f.truncate(0)Calling truncate() without a size uses the current position as the new length. Seeking to zero first is therefore essential; passing 0 explicitly makes the intended zero-byte length clear. Truncation does not change the stream’s current position. The Python I/O reference documents these stream behaviors.
Use the operating-system functions when appropriate
For direct path- or descriptor-based truncation, Python provides os.truncate(path, length) and os.ftruncate(fd, length). For example, os.truncate("example.txt", 0) sets the file length to zero. These functions are documented for Unix and Windows in the Python os module reference. The Python FAQ also shows truncation using an open descriptor: file truncation example.
Quick Recap
Check the stream and durability requirements
- A stream must be writable to write or truncate it, and seekable to use
seek(). Non-seekable streams cannot useseek(),tell(), ortruncate(); check the stream’s capabilities when working with unusual file-like objects. truncate(size)uses bytes, not a count of text characters. With text streams, use supported positions such as the start of the file rather than treating a character index as a byte offset.- Truncating a file changes its length; it is not the same as guaranteeing that buffered data has been forced to physical storage. If durability is required, flush a buffered file object before calling
os.fsync(f.fileno()).fsync()does not perform the truncation.
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