How do you append to a file?

Question

This can be achieved by setting the mode in open() to "a" (append) instead of "w" (write). See the documentation op open() for all available modes.

with open("test.txt", "a") as myfile:
    myfile.write("appended text")

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'

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

You probably want to pass "a" as the mode argument. See the docs for open().

with open("foo", "a") as f:
    f.write("cool beans...")

There are other permutations of the mode argument for updating (+), truncating (w) and binary (b) mode but starting with just "a" is your best bet.

You can also do it with print instead of write:

with open('test.txt', 'a') as f:
    print('appended text', file=f)

If test.txt doesn't exist, it will be created...