<file object>.truncate()
David Goodger
dgoodger at bigfoot.com
Sat May 20 22:29:37 EDT 2000
on 2000-05-20 19:36, Bjoern Lindstroem (magog at swipnet.se) wrote:
> When I do the following:
>
> f = open('file', 'w')
> f.truncate(3)
> f.close()
>
> the file becomes corrupted. It has the wished size, but it is all null
> bytes.
I assume that you want to open an existing file and truncate it, leaving the
existing (3 bytes of) data.
>From the Python Library Reference (1.5.2p2), section 2.8, Built-in
Functions:
open (filename[, mode[, bufsize]])
Return a new file object ... The first two arguments are the same
as for stdio's fopen(): ... mode indicates how the file is to be
opened: 'r' for reading, 'w' for writing (truncating an existing
file), and 'a' opens it for appending (which on some Unix systems
means that all writes append to the end of the file, regardless of
the current seek position).
Modes 'r+', 'w+' and 'a+' open the file for updating (note that
'w+' truncates the file). ...
I think your problem may be in your interpretation of the word "truncating"
under the 'w' mode above. Since Python's open() modes are the same as
stdio's fopen() in C, the following is relevant. From The C Programming
Language, Second Edition (Kernighan & Ritchie), Appendix B, section B1.1,
(<stdio.h>) File Operations:
FILE *fopen ...
... Legal values for mode include:
...
"w" create text file for writing; discard previous
contents if any
...
"w+" create text file for update; discard previous
contents if any
"a+" append; open or create text file for update, writing
at end
Update mode permits reading and writing the same file; ...
So in the Python docs, "truncating" is used, whereas in C, it's "discard
previous contents if any". A bit misleading. Basically, modes "w" and "w+"
both create a *new* file, clobbering/erasing any existing file with the same
name. So your f.truncate(3) makes perfect sense: it sets the file length of
a *new* file to 3 bytes, and since it *is* a new file, it's reasonable
(downright friendly, in my opinion!) to put nulls in there.
I think what you want is to use the mode "a+" for a text file, or "a+b" for
a binary file:
>>> f1 = open("tmpfile", "w")
>>> f1.write("123456789")
>>> f1.close()
>>> print open("tmpfile", "r").read()
123456789
>>> f2 = open("tmpfile", "a+")
>>> f2.truncate(3)
>>> f2.close()
>>> print open("tmpfile", "r").read()
123
--
David Goodger dgoodger at bigfoot.com Open-source projects:
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