How to ensure a file is closed for writing in Python?

The issue described here looked initially like it was solvable by just having the spreadsheet closed in Excel before running the program.

It transpires, however, that having Excel closed is a necessary, but not sufficient, condition. The issue still occurs, but not on every Windows machine, and not every time (sometimes it occurs after a single execution, sometimes two).

I've modified the program such that it now reads from one spreadsheet and writes to a different one, still the issue presents itself. I even go on to programmatically kill any lingering Python processes before running the program. Still no joy.

The openpyxl save() function instantiates ZipFile thus:

archive = ZipFile(filename, 'w', ZIP_DEFLATED, allowZip64=True)

... with Zipfile then using that to attempt to open the file in mode 'wb' thus:

if isinstance(file, basestring):
    self._filePassed = 0
    self.filename = file
    modeDict = {'r' : 'rb', 'w': 'wb', 'a' : 'r+b'}
    try:
        self.fp = open(file, modeDict[mode])
    except IOError:
        if mode == 'a':
            mode = key = 'w'
            self.fp = open(file, modeDict[mode])
        else:
            raise

According to the docs:

On Windows, 'b' appended to the mode opens the file in binary mode, so there are also modes like 'rb', 'wb', and 'r+b'. Python on Windows makes a distinction between text and binary files; the end-of-line characters in text files are automatically altered slightly when data is read or written. This behind-the-scenes modification to file data is fine for ASCII text files, but it’ll corrupt binary data like that in JPEG or EXE files. Be very careful to use binary mode when reading and writing such files. On Unix, it doesn’t hurt to append a 'b' to the mode, so you can use it platform-independently for all binary files.

... which explains why mode 'wb' must be used.

Is there something in Python file opening that could possibly leave the file in some state of "openness"?

Windows: 8

Python: 2.7.10

openpyxl: latest

Solution

Two suggestions:

First is to use with to close the file correctly.

with open("some.xls", "wb") as excel_file:
    #Do something

At the end of that the file will close on its own (see this).

You can also make a copy of the file and work on the copied file.

import shutil
shutil.copyfile(src, dst)

https://docs.python.org/2/library/shutil.html#shutil.copyfile