Often when working with files in Python, it is useful to be able to see the contents of the file as you are writing to it. This allows you to monitor how the data is being written and check for any errors. Python provides a few different ways to view the file contents while writing:
Opening the File in ‘Read/Write’ Mode
The simplest approach is to open the file in read/write mode using the ‘r+’ flag. This allows reading and writing to the file simultaneously.
To do this, open the file using:
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file = open(‘filename.txt’, ‘r+’)
You can then write to the file normally using file.write() and read from it using file.read() or file.readline().
For example:
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file = open(‘test.txt’, ‘r+’)
file.write(“This is the first line\n”)
print(file.read())
file.write(“This is the second line\n”)
print(file.read())
This will print out the contents of the file after each write. The downside is that each read will reset the file position, so subsequent writes may overwrite existing data.
Seeking to the Start After Writes
To avoid overwriting data, you need to seek back to the start of the file after each write using file.seek(0).
For example:
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file = open(‘test.txt’, ‘r+’)
file.write(“This is the first line\n”)
file.seek(0)
print(file.read())
file.write(“This is the second line\n”)
file.seek(0)
print(file.read())
Now it will print the full file contents each time without losing any data.
Using a Context Manager
For cleaner code, you can open the file using a context manager which will automatically close the file when done:
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with open(‘test.txt’, ‘r+’) as file:
file.write(“This is the first line\n”)
file.seek(0)
print(file.read())
Reading While Writing Using Temporary Files
For larger files, repeatedly seeking to the start can be inefficient. In these cases, you can write to a temporary file and read the original file simultaneously.
For example:
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import tempfile
with open(‘test.txt’) as original, open(tempfile.NamedTemporaryFile(), ‘w+’) as temp:
temp.write(“First line”)
print(original.read())
temp.write(“Second line”)
print(original.read())
# Rename temp file to original
temp.file.seek(0)
with open(‘test.txt’, ‘w’) as f:
f.write(temp.read())
This writes to a temporary file while printing the original file contents. At the end it overwrites the original file with the temp contents.
Reading Buffered Output
When writing large amounts of data, file.write() may buffer the output internally before flushing to the physical file. To see the real-time output, you can flush manually after writes:
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with open(‘test.txt’, ‘w’) as f:
f.write(“Line 1”)
f.flush()
print(open(‘test.txt’).read())
f.write(“Line 2”)
f.flush()
print(open(‘test.txt’).read())
The flush() forces any buffered data to be written immediately.
Reading While Appending
If you only need to append to the file and don’t need to read the existing contents first, you can open in append mode ‘a’ instead of read/write:
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with open(‘test.txt’, ‘a’) as f:
f.write(“Appended line 1\n”)
print(open(‘test.txt’).read())
f.write(“Appended line 2\n”)
print(open(‘test.txt’).read())
This is more efficient than seek() if you don’t need to reset position.
Using File Objects
For more advanced use cases, you can operate directly on the underlying file descriptor number using os.fsync().
This forces any buffered writes to disk immediately without closing the file:
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import os
file = open(‘test.txt’, ‘a+’)
file.write(“Line 1”)
os.fsync(file.fileno())
print(file.read())
file.write(“Line 2”)
os.fsync(file.fileno())
print(file.read())
This provides very low-level control over flushing.
Monitoring Files Externally
For truly monitoring the file in real-time as it is written, a better option may be to write from Python then view the file contents separately using an external tool like tail -f.
This avoids any issues from seeking, flushing or file locking inside Python itself.
For example:
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with open(‘test.txt’, ‘a’) as f:
f.write(“Line 1”)
f.write(“Line 2”)
In another terminal:
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tail -f test.txt
This provides an instantly updating view of the file contents as they are written without interfering with Python’s I/O.
Python provides several options for viewing file contents as they are written – opening in read/write mode and seeking, using temporary files, manually flushing, or monitoring externally. The best method depends on specific needs like performance, data volumes, and whether reading is needed before or after writes. With a bit of experimentation one approach can usually be found to fit each use case.