How to Append to a String in Python (Add Characters to a String)

To append a character or another string to a string in Python, use +=: word += "n". Python strings can’t be changed in place, so there is no append() method; + and += build a new string. To add to the beginning, put the new text first ("#" + s), to insert in the middle, slice the string, and to combine many pieces, use "".join().

word = "Pytho"
word += "n"            # append a character
print(word)

Output:

Python

Every example was run with Python 3.12.5; the output shown is from those runs.

Append with + and +=

greeting = "Hello"
name = "Ann"

message = greeting + ", " + name + "!"   # + joins strings into a new one
print(message)

Output:

Hello, Ann!

Add characters to an empty string

Start with "" and add to it in a loop:

result = ""                  # start with an empty string
for ch in "abc":
    result += ch.upper()     # add one character at a time
print(result, len(result))

Output:

ABC 3

To test whether a string is still empty, see check if a string is empty in Python.

Add a character to the beginning (prepend)

number = "42"
print("#" + number)          # add to the beginning
print(f"ID-{number}")        # or with an f-string

Output:

#42
ID-42

Insert a character at a specific position

Slice the string at the position and put the new character between the two parts:

code = "PY312"
position = 2
new_code = code[:position] + "-" + code[position:]   # insert "-" at index 2
print(new_code)

Output:

PY-312

More examples: insert a character into a string at an index.

Build a string with f-strings and format()

city = "Boston"
temp = 21.456

print(f"{city}: {temp:.1f}°C")               # f-string (Python 3.6+)
print("{}: {:.1f}°C".format(city, temp))      # str.format()
print("%s: %.1f°C" % (city, temp))            # older % formatting

Output:

Boston: 21.5°C
Boston: 21.5°C
Boston: 21.5°C

Join many pieces with join()

letters = ["P", "y", "t", "h", "o", "n"]
print("".join(letters))          # glue a list of strings together
print("-".join(letters))         # with a separator between each
print(", ".join(["red", "green", "blue"]))

Output:

Python
P-y-t-h-o-n
red, green, blue

See also concatenate a list of strings into one string.

Add a separator every N characters

digits = "1234567890"
grouped = " ".join(digits[i:i + 3] for i in range(0, len(digits), 3))
print(grouped)                   # add a space after every 3 characters

Output:

123 456 789 0

Repeat and pad characters

print("ab" * 3)                  # repeat a string
print("Title".center(15, "*"))   # add characters on both sides
print("7".zfill(3), "7".rjust(3, "0"), "7".ljust(3, "."))

Output:

ababab
*****Title*****
007 007 7..

More in create a string of N characters.

Add characters by their code with chr()

s = ""
for code in range(ord("a"), ord("e") + 1):
    s += chr(code)               # add characters by their Unicode code
print(s)

Output:

abcde

Why strings have no append()

Strings are immutable: once created, they never change. += looks like it changes the string, but it creates a new object and points the name at it. The original is untouched:

s = "cat"
original = s          # keep a second name for the original string
s += "s"              # builds a NEW string and points s at it
print(s, original, "| same object?", s is original)

Output:

cats cat | same object? False

That’s why the list-style methods fail:

s = "abc"
s.append("d")

Output:

AttributeError: 'str' object has no attribute 'append'

s = "abc"
s[0] = "X"

Output:

TypeError: 'str' object does not support item assignment

TypeError: can only concatenate str to str

count = 3
print("Items: " + count)

Output:

TypeError: can only concatenate str (not "int") to str

Convert the other value first, or use an f-string:

count = 3
print("Items: " + str(count))
print(f"Items: {count}")

Output:

Items: 3
Items: 3

Fastest way to build a long string

import timeit
from io import StringIO

N = 100_000

def plus_equals():
    s = ""
    for _ in range(N):
        s += "x"
    return s

def join_list():
    parts = []
    for _ in range(N):
        parts.append("x")
    return "".join(parts)

def string_io():
    buf = StringIO()
    for _ in range(N):
        buf.write("x")
    return buf.getvalue()

assert plus_equals() == join_list() == string_io()
for f in (plus_equals, join_list, string_io):
    t = min(timeit.repeat(f, number=1, repeat=5))
    print(f"{f.__name__:12} {t * 1000:6.1f} ms")

Output (building a 100,000-character string one character at a time):

plus_equals     7.8 ms
join_list       3.4 ms
string_io       4.0 ms

In this run, += took 7.8 ms, a list plus join() 3.4 ms and StringIO 4.0 ms. CPython speeds up s += x when nothing else refers to the string, so it is often fine in practice. That trick stops working as soon as another name points to the same string, because Python must then keep the old value:

a = "x"
b = a                # a second name for the same string
for _ in range(3):
    a += "y"         # b still refers to the old "x", so a must be copied
print(a, b)

Output:

xyyy x

For big loops, collecting the pieces in a list and calling "".join() once is the reliable, recommended way (see str.join() in the Python documentation).

Next, try these string tutorials:

• Insert a character into a string at a specific index
• Concatenate a list of strings into a single string
• Remove a character from a string in Python
• Check if a string is empty in Python

Frequently asked questions