io - Beginner Python: Reading and writing to the same file

Question

Started Python a week ago and I have some questions to ask about reading and writing to the same files. I've gone through some tutorials online but I am still confused about it. I can understand simple read and write files.

openFile = open("filepath", "r")
readFile = openFile.read()
print readFile 

openFile = open("filepath", "a")
appendFile = openFile.write("
Test 123")

openFile.close()

But, if I try the following I get a bunch of unknown text in the text file I am writing to. Can anyone explain why I am getting such errors and why I cannot use the same openFile object the way shown below.

# I get an error when I use the codes below:       
openFile = open("filepath", "r+")
writeFile = openFile.write("Test abc")

readFile = openFile.read()
print readFile

openFile.close()

I will try to clarify my problems. In the example above, openFile is the object used to open file. I have no problems if I want write to it the first time. If I want to use the same openFile to read files or append something to it. It doesn't happen or an error is given. I have to declare the same/different open file object before I can perform another read/write action to the same file.

#I have no problems if I do this:    
openFile = open("filepath", "r+")
writeFile = openFile.write("Test abc")

openFile2 = open("filepath", "r+")
readFile = openFile2.read()
print readFile

openFile.close()

I will be grateful if anyone can tell me what I did wrong here or is it just a Pythong thing. I am using Python 2.7. Thanks!

Answer 1

Updated Response:

This seems like a bug specific to Windows - http://bugs.python.org/issue1521491.

Quoting from the workaround explained at http://mail.python.org/pipermail/python-bugs-list/2005-August/029886.html

the effect of mixing reads with writes on a file open for update is entirely undefined unless a file-positioning operation occurs between them (for example, a seek()). I can't guess what you expect to happen, but seems most likely that what you intend could be obtained reliably by inserting

fp.seek(fp.tell())

between read() and your write().

My original response demonstrates how reading/writing on the same file opened for appending works. It is apparently not true if you are using Windows.

Original Response:

In 'r+' mode, using write method will write the string object to the file based on where the pointer is. In your case, it will append the string "Test abc" to the start of the file. See an example below:

>>> f=open("a","r+")
>>> f.read()
'Test abc
fasdfafasdfa
sdfgsd
'
>>> f.write("foooooooooooooo")
>>> f.close()
>>> f=open("a","r+")
>>> f.read()
'Test abc
fasdfafasdfa
sdfgsd
foooooooooooooo'

The string "foooooooooooooo" got appended at the end of the file since the pointer was already at the end of the file.

Are you on a system that differentiates between binary and text files? You might want to use 'rb+' as a mode in that case.

Append 'b' to the mode to open the file in binary mode, on systems that differentiate between binary and text files; on systems that don’t have this distinction, adding the 'b' has no effect. http://docs.python.org/2/library/functions.html#open