Append text to a file in Python ('a' mode)

Append mode ("a") adds new text to the end of a file. The text that is already in the file stays. This is the mode for logs, diaries and any file that grows over time.

The program

python

# start with a file that has two lines
with open("log.txt", "w", encoding="utf-8") as f:
    f.write("Ada logged in\n")
    f.write("Ada saved a file\n")

# later: add one more line to the end
with open("log.txt", "a", encoding="utf-8") as f:
    f.write("Ada logged out\n")

with open("log.txt", encoding="utf-8") as f:
    print(f.read(), end="")

output

Ada logged in
Ada saved a file
Ada logged out

How it works

"w" and "a" both create the file if it does not exist. The difference is what happens when it does exist: "w" empties it first, and "a" keeps the contents and writes after them.
In append mode, every write goes to the end of the file. You cannot use it to insert a line in the middle.
As with "w", you add the \n yourself. If the last line of the file has no \n, the new text joins that line.

The difference between "w" and "a"

This program runs the same write three times in each mode:

python

for mode in ["w", "a"]:
    with open("demo.txt", "w", encoding="utf-8"):
        pass  # start each test with an empty file
    for visit in range(3):
        with open("demo.txt", mode, encoding="utf-8") as f:
            f.write(f"visit {visit + 1}\n")
    with open("demo.txt", encoding="utf-8") as f:
        print(f"mode {mode!r}:", f.read().split("\n")[:-1])

output

mode 'w': ['visit 3']
mode 'a': ['visit 1', 'visit 2', 'visit 3']

With "w", each open deleted the previous line, so only the last visit is left.

Related programs: Write to a file.

Append to a file in Python

The program

How it works

The difference between "w" and "a"