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By definition, "a+" mode opens the file for both appending and reading. Appending works, but what is the method for reading? I did some searches, but couldn't find it clarified anywhere.

f=open("myfile.txt","a+")
print (f.read())

Tried this, it prints blank.

morris
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4 Answers4

24

Use f.seek() to set the file offset to the beginning of the file.

Note: Before Python 2.7, there was a bug that would cause some operating systems to not have the file position always point to the end of the file. This could cause some users to have your original code work. For example, on CentOS 6 your code would have worked as you wanted, but not as it should.

f = open("myfile.txt","a+")
f.seek(0)
print f.read()
Bill Lynch
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  • Thanks, it works. Maybe it would be good if I add 'windows' tag to my question, as it appears to be os related. – morris Feb 01 '13 at 05:14
  • No, The correct behavior would be that you would see no output. On some operating systems running pre 2.7 versions of python, it was possible to see output, because the file position was improperly set. – Bill Lynch Feb 01 '13 at 05:17
  • Sorry, you're telling me I shouldn't see the contents of my file when I execute that code on Win7 machine in Python3? – morris Feb 01 '13 at 05:30
  • You shouldn't see the contents of the file when you run the code you posted in the question. You should see the output when you run the code in this answer. – Bill Lynch Feb 01 '13 at 05:49
  • That's what I was saying, your code works.:) Thank you again. – morris Feb 01 '13 at 06:01
3

when you open the file using f=open(myfile.txt,"a+"), the file can be both read and written to.

By default the file handle points to the start of the file,

this can be determined by f.tell() which will be 0L.

In [76]: f=open("myfile.txt","a+")

In [77]: f.tell()
Out[77]: 0L

In [78]: f.read()
Out[78]: '1,2\n3,4\n'

However, f.write will take care of moving the pointer to the last line before writing.

avasal
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1

There are still quirks in newer version of Python dependant on OS and they are due to differences in implementation of the fopen() function in stdio.

Linux's man fopen:

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.

OS X:

``a+'' - Open for reading and writing. The file is created if it does not exist. The stream is positioned at the end of the file. Subsequent writes to the file will always end up at the then current end of file, irrespective of any intervening fseek(3) or similar.

MSDN doesn't really state where the pointer is initially set, just that it moves to the end on writes.

When a file is opened with the "a" or "a+" access type, all write operations occur at the end of the file. The file pointer can be repositioned using fseek or rewind, but is always moved back to the end of the file before any write operation is carried out. Thus, existing data cannot be overwritten.

Replicating the differences on various systems with both Python 2.7.x and 3k are pretty straightforward with .open .tell

When dealing with anything through the OS, it's safer to take precautions like using an explicit .seek(0).

m.brindley
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0

MODES r+ read and write Starts at the beginning of the file r read only Starts at the beginning of the file a+ Read/Append. Preserves file content by writing to the end of the file

Good Luck! Isabel Ruiz

IsabelRuizN
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