in a py module, I write:
outFile = open(fileName, mode='w')
if A:
outFile.write(...)
if B:
outFile.write(...)
and in these lines, I didn't use flush or close method. Then after these lines, I want to check whether this "outFile" object is empty or not. How can I do with it?
There are a few problems with your code.
You can't .write to a file that you opened with 'r'. You need to open(fileName, 'w').
If A or B then you've certainly written to the file, so it's not empty!
Barring those. you can get the length of a file with
os.stat(outFile.fileno())
EDIT: I'll explain what flush does. Python is often used to do quite large amounts of file reads and writes, which can be slow. It is thus tweaked to make them as fast as possible. One way that is does so is to "buffer" such writes and then do them all in one big block: when you write a small string, Python will remember it but won't actually write it to the file until it thinks it should.
This means that if you want to tell whether you have written data to the file by inspecting the file, you have to tell Python to write all the data it's remembering first, or else you might not see it. flush is the command to write all the buffered data.
Of course, if you ask Python whether it's written anything to the file, say by inspecting the position in the file (.tell()), then it will know about the buffering.
outFile.write('') didn't write a thing!
st_size is nonzero yet. I just did a test, writing "z" to a file and then calling os.stat immediately, and got st_size=0. [P.S. I think os.stat accepts a path, not a fileno.]
If you've already written to the file, you can use .tell() to check if the current file position is nonzero:
>>> handle = open('/tmp/file.txt', 'w')
>>> handle.write('foo')
>>> handle.tell()
3
This won't work if you .seek() back to the beginning of the file.
with open("filename.txt", "r+") as f:
if f.read():
# file isn't empty
f.write("something")
# uncomment this line if you want to delete everything else in the file
# f.truncate()
else:
# file is empty
f.write("somethingelse")
"r+" mode always you to read & write. "with" will automatically close file
'r', you can't usewrite().