File mode for creating+reading+appending+binary

Question

I need to open a file for reading and writing. If the file is not found, it should be created. It should also be treated as a binary for Windows. Can you tell me the file mode sequence I need to use for this?

I tried ‘r+ab’ but that doesn’t create the files if they are not found.

Thanks

Asked By: Mihai Damian

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Source

Answer 1

open("filename", "a+b")

should work. It opens a binary file in append/update mode.

Answered By: Tim Pietzcker

Answer 2

The mode is ab+ the r is implied and ‘a’ppend and (‘w’rite ‘+’ ‘r’ead) are redundant. Since the CPython (i.e. regular python) file is based on the C stdio FILE type, here are the relevant lines from the fopen(3) man page:

w+ Open for reading and writing.
The file is created if it does not
exist, otherwise it is truncated.
The stream is positioned at the
beginning of the file.
a+ Open for reading and appending (writing at end of file).
The file is created if it does not
exist. The initial file position
for reading is at the beginning of
the file, but output is always
appended to the end of the file.

With the “b” tacked on to make DOS happy. Presumably you want to do something like this:

>>> f = open('junk', 'ab+')
>>> f
<open file 'junk', mode 'ab+' at 0xb77e6288>
>>> f.write('hellon')
>>> f.seek(0, os.SEEK_SET)
>>> f.readline()
'hellon'
>>> f.write('theren')
>>> f.seek(0, os.SEEK_SET)
>>> f.readline()
'hellon'
>>> f.readline()
'theren'

Answered By: msw

File mode for creating+reading+appending+binary

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