[python] How do you append to a file?


Answers

You need to open the file in append mode, by setting "a" or "ab" as the mode. See open().

When you open with "a" mode, the write position will always be at the end of the file (an append). You can open with "a+" to allow reading, seek backwards and read (but all writes will still be at the end of the file!).

Example:

>>> with open('test1','wb') as f:
        f.write('test')
>>> with open('test1','ab') as f:
        f.write('koko')
>>> with open('test1','rb') as f:
        f.read()
'testkoko'

Note: Using 'a' is not the same as opening with 'w' and seeking to the end of the file - consider what might happen if another program opened the file and started writing between the seek and the write. On some operating systems, opening the file with 'a' guarantees that all your following writes will be appended atomically to the end of the file (even as the file grows by other writes).


A few more details about how the "a" mode operates (tested on Linux only). Even if you seek back, every write will append to the end of the file:

>>> f = open('test','a+') # Not using 'with' just to simplify the example REPL session
>>> f.write('hi')
>>> f.seek(0)
>>> f.read()
'hi'
>>> f.seek(0)
>>> f.write('bye') # Will still append despite the seek(0)!
>>> f.seek(0)
>>> f.read()
'hibye'

In fact, the fopen manpage states:

Opening a file in append mode (a as the first character of mode) causes all subsequent write operations to this stream to occur at end-of-file, as if preceded the call:

fseek(stream, 0, SEEK_END);

Old simplified answer (not using with):

Example: (in a real program use with to close the file - see the documentation)

>>> open("test","wb").write("test")
>>> open("test","a+b").write("koko")
>>> open("test","rb").read()
'testkoko'
Question

How do you append to the file instead of overwriting it? Is there a special function that appends to the file?




Here's my script, which basically counts the number of lines, then appends, then counts them again so you have evidence it worked.

shortPath  = "../file_to_be_appended"
short = open(shortPath, 'r')

## this counts how many line are originally in the file:
long_path = "../file_to_be_appended_to" 
long = open(long_path, 'r')
for i,l in enumerate(long): 
    pass
print "%s has %i lines initially" %(long_path,i)
long.close()

long = open(long_path, 'a') ## now open long file to append
l = True ## will be a line
c = 0 ## count the number of lines you write
while l: 
    try: 
        l = short.next() ## when you run out of lines, this breaks and the except statement is run
        c += 1
        long.write(l)

    except: 
        l = None
        long.close()
        print "Done!, wrote %s lines" %c 

## finally, count how many lines are left. 
long = open(long_path, 'r')
for i,l in enumerate(long): 
    pass
print "%s has %i lines after appending new lines" %(long_path, i)
long.close()



I always do this,

f = open('filename.txt', 'a')
f.write("stuff")
f.close()

It's simple, but very useful.




Python has many variations off of the main three modes, these three modes are:

'w'   write text
'r'   read text
'a'   append text

So to append to a file it's as easy as:

f = open('filename.txt', 'a') 
f.write('whatever you want to write here (in append mode) here.')

Then there are the modes that just make your code fewer lines:

'r+'  read + write text
'w+'  read + write text
'a+'  append + read text

Finally, there are the modes of reading/writing in binary format:

'rb'  read binary
'wb'  write binary
'ab'  append binary
'rb+' read + write binary
'wb+' read + write binary
'ab+' append + read binary



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