Re: problem with read() write()
Alf P. Steinbach wrote: So with 'w+' the only way to get garbage is if 'read' reads beyond the end of file, or 'open' doesn't conform to the documentation. It does read beyond the end of file. This is perhaps the way the underlying C library works, but it looks like an unexpected feature (read: bug) to me. I reproduced (with Python 2.5.2 on WinXP) the code the OP wrote after creating an empty (0-byte) test file; after the write() the read() returns random garbage. I can't imagine why anyone would want that behaviour. The file grew to be 4099 bytes after f.close(). I wrote 'hello' to it, so the length of garbage added was 4094 bytes, which I find a strange number also. I would have expected the read to return nothing. Can anyone explain or even defend this behaviour? Gertjan. -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
* Gertjan Klein: Alf P. Steinbach wrote: So with 'w+' the only way to get garbage is if 'read' reads beyond the end of file, or 'open' doesn't conform to the documentation. It does read beyond the end of file. This is perhaps the way the underlying C library works, but it looks like an unexpected feature (read: bug) to me. I reproduced (with Python 2.5.2 on WinXP) the code the OP wrote after creating an empty (0-byte) test file; after the write() the read() returns random garbage. I can't imagine why anyone would want that behaviour. The file grew to be 4099 bytes after f.close(). I wrote 'hello' to it, so the length of garbage added was 4094 bytes, which I find a strange number also. Could you post (copy and paste) the code, and description of results? I would have expected the read to return nothing. Can anyone explain or even defend this behaviour? I'm just a Python newbie, but in C and C++ such things are usually down to undefined behavior, that is, the program doing something that is implicitly or explicitly defined as undefined behavior by the language standard. With UB the effect may then be something or nothing or anything or just what you expected; appearance of the infamous nasal demons is one possibility... Quoting n869, which is the January 18th 1999 draft of the C99 standard: §7.19.5.3/6 When a file is opened with update mode (’+’ as the second or third character in the above list of mode argument values), both input and output may be performed on the associated stream. However, output shall not be directly followed by input without an intervening call to the fflush function or to a file positioning function (fseek, fsetpos, or rewind), and input shall not be directly followed by output without an intervening call to a file positioning function, unless the input operation encounters end-of-file. Opening( or creating) a text file with update mode may instead open (or create) a binary stream in some implementations. Shall not means UB. This applies to C FILE* handling. AFAICS nothing except efficiency prevents the Python wrapper, if FILE* is what it uses, from automatically inserting an appropriate fflush or fseek. And for a language used by so many newbies (this is positive!) I agree that it should ideally get rid of that UB (assuming that's what the problem is), or, if it doesn't already, mention that in the Python documentation. Cheers, - Alf -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
Alf P. Steinbach wrote: * Gertjan Klein: I reproduced (with Python 2.5.2 on WinXP) the code the OP wrote after creating an empty (0-byte) test file; after the write() the read() returns random garbage. I can't imagine why anyone would want that behaviour. The file grew to be 4099 bytes after f.close(). I wrote 'hello' to it, so the length of garbage added was 4094 bytes, which I find a strange number also. Could you post (copy and paste) the code, and description of results? The code is exactly the OP's code, with an f.close() after f.read(). The results are described above. Shall not means UB. This applies to C FILE* handling. Yes. But Python is not C. I would have expected that Python shields me from such bizarre results. The fact that, apparently, the Python file object is a thin wrapper over a C library function is an explanation of the behaviour, but not a justification. And for a language used by so many newbies (this is positive!) I agree that it should ideally get rid of that UB (assuming that's what the problem is), or, if it doesn't already, mention that in the Python documentation. I'm not sure I can defend this statement, but I think Python should not have undefined behaviour at all. Gertjan. -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
Gertjan Klein wrote: Alf P. Steinbach wrote: So with 'w+' the only way to get garbage is if 'read' reads beyond the end of file, or 'open' doesn't conform to the documentation. It does read beyond the end of file. This is perhaps the way the underlying C library works, but it looks like an unexpected feature (read: bug) to me. I reproduced (with Python 2.5.2 on WinXP) the code the OP wrote after creating an empty (0-byte) test file; after the write() the read() returns random garbage. I can't imagine why anyone would want that behaviour. The file grew to be 4099 bytes after f.close(). I wrote 'hello' to it, so the length of garbage added was 4094 bytes, which I find a strange number also. I would have expected the read to return nothing. Can anyone explain or even defend this behaviour? Gertjan. I wonder, does it still happen if you open the file in binary mode? (Stick a b in the file mode.) It could be some Windows text mode craziness. -- -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
On Sun, 01 Nov 2009 10:44:45 +0100, Alf P. Steinbach wrote:
Could you post (copy and paste) the code, and description of results?
Using Python 2.6 under Linux (Fedora 7):
f = open('garbage', 'r') # prove the file doesn't exist
Traceback (most recent call last):
File stdin, line 1, in module
IOError: [Errno 2] No such file or directory: 'garbage'
f = open('garbage', 'wb+')
f.read(4)
''
f.write('hello world\n')
f.seek(0)
f.read(100)
'hello world\n'
--
Steven
--
http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
* Zeynel: Hello, I've been studying the official tutorial, so far it's been fun, but today I ran into a problem with the write(). So, I open the file pw and write hello and read: f = open(pw, r+) f.write(hello) f.read() But read() returns a bunch of what looks like meta code: ont': 1, 'center_insert_even\xc4\x00K\x02\xe8\xe1[\x02z\x8e \xa5\x02\x0b \x00\x00\x00\x00\x00\x00\x00\x08\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00'QUEUE'\np1\n (S'exec' What am I doing wrong? Thank you. After the 'write' the current position in the file is after the hello, so reading will read further content from there. The following works (disclaimer: I'm utter newbie in Python, and didn't consult the documentation, and it's the first time I've seen the Python 'open'): f = open(pw, r+) f.write( hello ) f.seek( 0 ) # Go back to start of file f.read() f.close() Cheers hth., - Alf -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
On Oct 31, 9:23 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: Hello, I've been studying the official tutorial, so far it's been fun, but today I ran into a problem with the write(). So, I open the file pw and write hello and read: f = open(pw, r+) f.write(hello) f.read() But read() returns a bunch of what looks like meta code: ont': 1, 'center_insert_even\xc4\x00K\x02\xe8\xe1[\x02z\x8e \xa5\x02\x0b \x00\x00\x00\x00\x00\x00\x00\x08\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0 0'QUEUE'\np1\n (S'exec' What am I doing wrong? Thank you. After the 'write' the current position in the file is after the hello, so reading will read further content from there. The following works (disclaimer: I'm utter newbie in Python, and didn't consult the documentation, and it's the first time I've seen the Python 'open'): f = open(pw, r+) f.write( hello ) f.seek( 0 ) # Go back to start of file f.read() f.close() Cheers hth., - Alf Thanks, but it didn't work for me. I still get the meta file. Although I see that hello is there. -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
* Zeynel: On Oct 31, 9:23 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: Hello, I've been studying the official tutorial, so far it's been fun, but today I ran into a problem with the write(). So, I open the file pw and write hello and read: f = open(pw, r+) f.write(hello) f.read() But read() returns a bunch of what looks like meta code: ont': 1, 'center_insert_even\xc4\x00K\x02\xe8\xe1[\x02z\x8e \xa5\x02\x0b \x00\x00\x00\x00\x00\x00\x00\x08\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0 0'QUEUE'\np1\n (S'exec' What am I doing wrong? Thank you. After the 'write' the current position in the file is after the hello, so reading will read further content from there. The following works (disclaimer: I'm utter newbie in Python, and didn't consult the documentation, and it's the first time I've seen the Python 'open'): f = open(pw, r+) f.write( hello ) f.seek( 0 ) # Go back to start of file f.read() f.close() Cheers hth., - Alf Thanks, but it didn't work for me. I still get the meta file. Although I see that hello is there. Just a thought: try w+ instead of r+. Because if you do print( open.__doc__ ) as I recall it said something about w truncating the file? Cheers hth., - Alf -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
On Oct 31, 9:55 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: On Oct 31, 9:23 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: Hello, I've been studying the official tutorial, so far it's been fun, but today I ran into a problem with the write(). So, I open the file pw and write hello and read: f = open(pw, r+) f.write(hello) f.read() But read() returns a bunch of what looks like meta code: ont': 1, 'center_insert_even\xc4\x00K\x02\xe8\xe1[\x02z\x8e \xa5\x02\x0b \x00\x00\x00\x00\x00\x00\x00\x08\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0 0'QUEUE'\np1\n (S'exec' What am I doing wrong? Thank you. After the 'write' the current position in the file is after the hello, so reading will read further content from there. The following works (disclaimer: I'm utter newbie in Python, and didn't consult the documentation, and it's the first time I've seen the Python 'open'): f = open(pw, r+) f.write( hello ) f.seek( 0 ) # Go back to start of file f.read() f.close() Cheers hth., - Alf Thanks, but it didn't work for me. I still get the meta file. Although I see that hello is there. Just a thought: try w+ instead of r+. Because if you do print( open.__doc__ ) as I recall it said something about w truncating the file? Cheers hth., - Alf No :) I still got the same thing. -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
* Zeynel: On Oct 31, 9:55 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: On Oct 31, 9:23 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: Hello, I've been studying the official tutorial, so far it's been fun, but today I ran into a problem with the write(). So, I open the file pw and write hello and read: f = open(pw, r+) f.write(hello) f.read() But read() returns a bunch of what looks like meta code: ont': 1, 'center_insert_even\xc4\x00K\x02\xe8\xe1[\x02z\x8e \xa5\x02\x0b \x00\x00\x00\x00\x00\x00\x00\x08\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0 0'QUEUE'\np1\n (S'exec' What am I doing wrong? Thank you. After the 'write' the current position in the file is after the hello, so reading will read further content from there. The following works (disclaimer: I'm utter newbie in Python, and didn't consult the documentation, and it's the first time I've seen the Python 'open'): f = open(pw, r+) f.write( hello ) f.seek( 0 ) # Go back to start of file f.read() f.close() Cheers hth., - Alf Thanks, but it didn't work for me. I still get the meta file. Although I see that hello is there. Just a thought: try w+ instead of r+. Because if you do print( open.__doc__ ) as I recall it said something about w truncating the file? Cheers hth., - Alf No :) I still got the same thing. Hm. Now I had to look in the docs because I thought I'd given bad advice. Doc of 'open' says: The mode 'w+' opens and truncates the file to 0 bytes, while 'r+' opens the file without truncation. So with 'w+' the only way to get garbage is if 'read' reads beyond the end of file, or 'open' doesn't conform to the documentation. Testing with Python 3.1.1 under Windows XP Pro: example f = open( zilly, w+ ) f.write( garbagegarbagegarbagegarbagegarbagegarbagegarbagegarbagegarbage ) 63 f.close() f = open( zilly, r ) f.read() 'garbagegarbagegarbagegarbagegarbagegarbagegarbagegarbagegarbage' f.close() f.open( zilly, r+ ) Traceback (most recent call last): File stdin, line 1, in module AttributeError: '_io.TextIOWrapper' object has no attribute 'open' open( zilly, r+ ) _io.TextIOWrapper name='zilly' encoding='cp1252' f = open( zilly, r+ ) f.write( hello ) 5 f.seek( 0 ) 0 f.read() 'hellogegarbagegarbagegarbagegarbagegarbagegarbagegarbagegarbage' f.close() f = open( zilly, w+ ) f.write( hello ) 5 f.seek( 0 ) 0 f.read() 'hello' f.close() /example The w+ works here. Even if I made a typing mistake and apparently left the file open in the middle there. Cheers hth., - Alf -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
Zeynel wrote: On Oct 31, 9:55 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: On Oct 31, 9:23 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: Hello, I've been studying the official tutorial, so far it's been fun, but today I ran into a problem with the write(). So, I open the file pw and write hello and read: f =pen(pw, r+) f.write(hello) f.read() But read() returns a bunch of what looks like meta code: ont': 1, 'center_insert_even\xc4\x00K\x02\xe8\xe1[\x02z\x8e \xa5\x02\x0b \x00\x00\x00\x00\x00\x00\x00\x08\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0 0'QUEUE'\np1\n (S'exec' What am I doing wrong? Thank you. After the 'write' the current position in the file is after the hello, so reading will read further content from there. The following works (disclaimer: I'm utter newbie in Python, and didn't consult the documentation, and it's the first time I've seen the Python 'open'): f =pen(pw, r+) f.write( hello ) f.seek( 0 ) # Go back to start of file f.read() f.close() Cheers hth., - Alf Thanks, but it didn't work for me. I still get the meta file. Although I see that hello is there. Just a thought: try w+ instead of r+. Because if you do print( open.__doc__ ) as I recall it said something about w truncating the file? Cheers hth., - Alf No :) I still got the same thing. When you ran the program, the file already existed. So you're mixing the old content and the new. If you don't want that, then don't use r+. When testing, you should start with the system in a known state. Try initializing (with a text editor for example) that file before trying the program. And if it still gives you problems, show us the data in the file before running your code, your new code, and the results you get. One thing that'd be useful is to actually assign the results of the read() function to a variable and print it. DaveA -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
On Oct 31, 10:40 am, Alf P. Steinbach al...@start.no wrote: Thanks! This works. But I need to close the file before read and open it again with r, otherwise I get the garbage again. Can you give me the link where you got this in documentation: The mode 'w+' opens and truncates the file to 0 bytes, while 'r+' opens the file without truncation. Only place i could find it was in this bug report: http://bugs.python.org/issue5061 * Zeynel: On Oct 31, 9:55 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: On Oct 31, 9:23 am, Alf P. Steinbach al...@start.no wrote: * Zeynel: Hello, I've been studying the official tutorial, so far it's been fun, but today I ran into a problem with the write(). So, I open the file pw and write hello and read: f = open(pw, r+) f.write(hello) f.read() But read() returns a bunch of what looks like meta code: ont': 1, 'center_insert_even\xc4\x00K\x02\xe8\xe1[\x02z\x8e \xa5\x02\x0b \x00\x00\x00\x00\x00\x00\x00\x08\x00\x00\x00\x00\x00\x00\x00\x00\x00\x00\x0 0'QUEUE'\np1\n (S'exec' What am I doing wrong? Thank you. After the 'write' the current position in the file is after the hello, so reading will read further content from there. The following works (disclaimer: I'm utter newbie in Python, and didn't consult the documentation, and it's the first time I've seen the Python 'open'): f = open(pw, r+) f.write( hello ) f.seek( 0 ) # Go back to start of file f.read() f.close() Cheers hth., - Alf Thanks, but it didn't work for me. I still get the meta file. Although I see that hello is there. Just a thought: try w+ instead of r+. Because if you do print( open.__doc__ ) as I recall it said something about w truncating the file? Cheers hth., - Alf No :) I still got the same thing. Hm. Now I had to look in the docs because I thought I'd given bad advice. Doc of 'open' says: The mode 'w+' opens and truncates the file to 0 bytes, while 'r+' opens the file without truncation. So with 'w+' the only way to get garbage is if 'read' reads beyond the end of file, or 'open' doesn't conform to the documentation. Testing with Python 3.1.1 under Windows XP Pro: example f = open( zilly, w+ ) f.write( garbagegarbagegarbagegarbagegarbagegarbagegarbagegarbagegarbage ) 63 f.close() f = open( zilly, r ) f.read() 'garbagegarbagegarbagegarbagegarbagegarbagegarbagegarbagegarbage' f.close() f.open( zilly, r+ ) Traceback (most recent call last): File stdin, line 1, in module AttributeError: '_io.TextIOWrapper' object has no attribute 'open' open( zilly, r+ ) _io.TextIOWrapper name='zilly' encoding='cp1252' f = open( zilly, r+ ) f.write( hello ) 5 f.seek( 0 ) 0 f.read() 'hellogegarbagegarbagegarbagegarbagegarbagegarbagegarbagegarbage' f.close() f = open( zilly, w+ ) f.write( hello ) 5 f.seek( 0 ) 0 f.read() 'hello' f.close() /example The w+ works here. Even if I made a typing mistake and apparently left the file open in the middle there. Cheers hth., - Alf -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
Zeynel wrote: On Oct 31, 10:40 am, Alf P. Steinbach al...@start.no wrote: Thanks! This works. But I need to close the file before read and open it again with r, otherwise I get the garbage again. Can you give me the link where you got this in documentation: The mode 'w+' opens and truncates the file to 0 bytes, while 'r+' opens the file without truncation. Only place i could find it was in this bug report: http://bugs.python.org/issue5061 LibRef / builtin functions /open The entry is a full page. Get familiar with this section and the Built-in Types section. Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
Re: problem with read() write()
On Oct 31, 3:11 pm, Terry Reedy tjre...@udel.edu wrote: Great, thanks. Zeynel wrote: On Oct 31, 10:40 am, Alf P. Steinbach al...@start.no wrote: Thanks! This works. But I need to close the file before read and open it again with r, otherwise I get the garbage again. Can you give me the link where you got this in documentation: The mode 'w+' opens and truncates the file to 0 bytes, while 'r+' opens the file without truncation. Only place i could find it was in this bug report:http://bugs.python.org/issue5061 LibRef / builtin functions /open The entry is a full page. Get familiar with this section and the Built-in Types section. Terry Jan Reedy -- http://mail.python.org/mailman/listinfo/python-list
