PyPy file append mode

Question

I have code like this:

f1 = open('file1', 'a')
f2 = open('file1', 'a')

f1.write('Test line 1\n')
f2.write('Test line 2\n')
f1.write('Test line 3\n')
f2.write('Test line 4\n')

When this code is run with standard Python 2.7 interpreter, the file contains four lines as expected. However, when I run this code under PyPy, the file contains only two lines.

Could someone explain the differences between Python and PyPy in working with files in append mode?

UPDATED: The problem doesn't exist in the PyPy 2.3.

Answer 1

The reason in different behavior is different implementation of file I/O operations.

CPython implements it's file I/O on top of fopen, fread and fwrite functions from stdio.h and is working with FILE * streams.

In the same time PyPy implements it's file I/O on top of POSIX open, write and read functions and is working with int file descriptors.

Compare these two programs in C:

#include <stdio.h>

int main() {
    FILE *a = fopen("file1", "a");
    FILE *b = fopen("file1", "a");

    fwrite("Test line 1\n", 12, 1, a);
    fflush(a);
    fwrite("Test line 2\n", 12, 1, b);
    fflush(b);
    fwrite("Test line 3\n", 12, 1, a);
    fflush(a);
    fwrite("Test line 4\n", 12, 1, b);

    fclose(a);
    fclose(b);

    return 0;
}

and

#include <fcntl.h>
#include <unistd.h>

int main() {
    int a = open("file1", O_CREAT | O_WRONLY | O_APPEND);
    int b = open("file1", O_CREAT | O_WRONLY | O_APPEND);

    write(a, "Test line 1\n", 12);
    write(b, "Test line 2\n", 12);
    write(a, "Test line 3\n", 12);
    write(b, "Test line 4\n", 12);

    close(a);
    close(b);

    return 0;
}

More info on difference between open and fopen you could find in answers to this question.

UPDATE:

After inspecting PyPy codebase some more, it seems to me it doesn't use O_APPEND flag by some reason, but O_WRONLY | O_CREAT for "a" mode. So it is the real reason in PyPy you need to seek to the end of file after each write call, as J.F. Sebastian mentioned in another answer. I guess a bug should be created at PyPy bugtracker, as O_APPEND flag is available both on Windows and Unix. So, what PyPy does now looks like:

#include <fcntl.h>
#include <unistd.h>

int main() {
    int a = open("file1", O_CREAT | O_WRONLY);
    int b = open("file1", O_CREAT | O_WRONLY);

    write(a, "Test line 1\n", 12);
    write(b, "Test line 2\n", 12);
    write(a, "Test line 3\n", 12);
    write(b, "Test line 4\n", 12);

    close(a);
    close(b);

    return 0;
}

Without O_APPEND flag it should reproduce PyPy behavior.

Answer 2

On POSIX systems:

O_APPEND
    If set, the file offset shall be set to the end of the file prior to each write.

It means that if a file is opened in "append" mode then when its buffer is flushed; the content shall go to the end of the file.

Python 2, Python 3, Jython respect that on my machine. In your case, the content is smaller than the file buffer therefore you see all writes from one file followed by all writes from another file in the result on the disk.

It is easier to understand if the files are line-buffered:

from __future__ import with_statement

filename = 'file1'
with open(filename, 'wb', 0) as file:
    pass # truncate the file

f1 = open(filename, 'a', 1)
f2 = open(filename, 'a', 1)

f1.write('f1 1\n')
f2.write('f2 aa\n')
f1.write('f1 222\n')
f2.write('f2 bbbb\n')
f1.write('f1 333\n')
f2.write('f2 cc\n')

Output

f1 1
f2 aa
f1 222
f2 bbbb
f1 333
f2 cc

Python documentation does not mandate such behaviour. It just mentions:

..'a' for appending (which on some Unix systems means that all writes append to the end of the file regardless of the current seek position)^{emphasize is mine}

Pypy produces the following output in unbuffered and line-buffered mode:

f2 aaff2 bbbf1f2 cc

Manually moving the file position to the end fixes pypy output:

from __future__ import with_statement
import os

filename = 'file1'
with open(filename, 'wb', 0) as file:
    pass # truncate the file

f1 = open(filename, 'a', 1)
f2 = open(filename, 'a', 1)

f1.write('f1 1\n')
f2.seek(0, os.SEEK_END)
f2.write('f2 aa\n')
f1.seek(0, os.SEEK_END)
f1.write('f1 222\n')
f2.seek(0, os.SEEK_END)
f2.write('f2 bbbb\n')
f1.seek(0, os.SEEK_END)
f1.write('f1 333\n')
f2.seek(0, os.SEEK_END)
f2.write('f2 cc\n')

If the file is fully-buffered then add .flush() after each .write().

It is probably not a good idea to write to the same file using more than one file object at once.