How do I append to a file?

Set the mode in open() to "a" (append) instead of "w" (write):

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

The documentation lists all the available modes.

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.

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.