TypeError: write() argument must be str, not X in Python

# Table of Contents

# TypeError: write() argument must be str, not bytes

The Python "TypeError: write() argument must be str, not bytes" occurs when we try to write bytes to a file without opening the file in wb mode.

To solve the error, open the file in wb mode to write bytes or decode the bytes object into a string.

typeerror write argument must be str not bytes

Here is an example of how the error occurs.

main.py

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with open('example.txt', 'w') as my_file:
    # ⛔️ TypeError: write() argument must be str, not bytes
    my_file.write(b'bobbyhadz.com')

We opened the file in w mode and tried writing bytes to it which caused the error.

The string literal in the example is prefixed with b, so it is a bytes object.

# Open the file in `wb` (write binary) mode to write bytes

If you need to write bytes to a file, open it in wb (write binary) mode rather than w (write text) mode.

main.py

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# ✅ Open file in `wb` mode
with open('example.txt', 'wb') as my_file:
    my_file.write(b'bobbyhadz.com')

writing bytes to a file

We opened the file in wb (binary) mode to write bytes to it.

Note that you shouldn't specify the encoding keyword argument when you open a file in binary mode.

If you need to also read from the file, use the rb (read binary) mode.

main.py

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# ✅ Writing to the file
with open('example.txt', 'wb') as my_file:
    my_file.write(b'bobbyhadz.com')

# ✅ Reading from the file
with open('example.txt', 'rb') as my_file:
    lines = my_file.readlines()
    print(lines) # 👉️ [b'bobbyhadz.com']

    first_line = lines[0].decode('utf-8')

    print(first_line)  # 👉️ bobbyhadz.com

read from file using rb mode

The readlines() method returns a list of bytes objects, so we used the decode() method to convert the first item in the list to a string.

You can use a list comprehension to convert all lines in the file to strings.

main.py

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with open('example.txt', 'wb') as my_file:
    my_file.write(b'bobbyhadz.com\n')
    my_file.write(b'bobbyhadz.com\n')

with open('example.txt', 'rb') as my_file:
    lines = [l.decode('utf-8') for l in my_file.readlines()]

    # 👇️ ['bobbyhadz.com\n', 'bobbyhadz.com\n']
    print(lines)

using list comprehension to convert all lines to strings

List comprehensions are used to perform some operation for every element or select a subset of elements that meet a condition.

# Open the file in `w` (write text) mode to write text

Alternatively, you can decode the bytes object into a string.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_file.write(b'bobbyhadz.com'.decode('utf-8'))

open file in write text mode

Notice that we used the bytes.decode() method to decode the bytes object into a string.

main.py

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my_bytes = 'apple'.encode()
print(my_bytes) # 👉️ b'apple'

my_str = my_bytes.decode('utf-8')
print(my_str) # 👉️ 'apple'

We opened the JSON file in w (write text) mode.

When a file is opened in text mode, we read and write strings from and to the file.

If you need to read from the file, open it in r (read text) mode.

main.py

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# ✅ Writing to the file
with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_file.write(b'bobbyhadz.com'.decode('utf-8'))

# ✅ Reading from the file
with open('example.txt', 'r', encoding='utf-8') as my_file:
    lines = my_file.readlines()
    print(lines) # 👉️ ['bobbyhadz.com']

Those strings are encoded using a specific encoding (utf-8 in the example).

If you don't explicitly set the encoding keyword argument, the default is platform-dependent.

If you want to both read from and write to the file, use the r+ mode when opening it.

If you append a b to the mode (like in the first code sample), the file is opened in binary mode.

Note that you cannot specify encoding when opening a file in binary mode.

When a file is opened in binary mode, data is read and written as bytes objects.

The bytes.decode() method returns a string decoded from the given bytes. The default encoding is utf-8.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_file.write(b'hello world'.decode('utf-8'))

You can decode your bytes object to a string if you want to write strings to the file.

Conversely, the str.encode() method returns an encoded version of the string as a bytes object. The default encoding is utf-8.

main.py

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with open('example.txt', 'wb') as my_file:
    my_file.write('hello world'.encode('utf-8'))

The example shows how to encode a string to a bytes object and write the bytes to a file.

Encoding is the process of converting a string to a bytes object and decoding is the process of converting a bytes object to a string.

# Table of Contents

# TypeError: write() argument must be str, not list

The Python "TypeError: write() argument must be str, not list" occurs when we try to write a list object to a file using the write() method.

To solve the error, use the join() method to join the list into a string.

typeerror write argument must be str not list

Here is an example of how the error occurs.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_list = ['Alice', 'Bob', 'Carl']

    # ⛔️ TypeError: write() argument must be str, not list
    my_file.write(my_list)

We passed a list object to the write() method, but the method can only be called with a string argument.

# Use the `str.join()` method to join the list into a string

One way to solve the error is to use the str.join() method to join the list into a string.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_list = ['Alice', 'Bob', 'Carl']

    my_file.write(','.join(my_list)) # 👉️ "Alice,Bob,Carl"

We used a comma as the separator between the items of the list, but you can use any other separator.

main.py

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my_list = ['Alice', 'Bob', 'Carl']

print(' '.join(my_list))  # 👉️ Alice Bob Carl
print('-'.join(my_list))  # 👉️ Alice-Bob-Carl
print(''.join(my_list))  # 👉️ AliceBobCarl

You can also use an empty string if you don't need a separator between the elements.

The str.join() method takes an iterable as an argument and returns a string which is the concatenation of the strings in the iterable.

Note that the method raises a TypeError if there are any non-string values in the iterable.

# All values in the collection must be strings

If your list contains numbers or other types, convert all of the values to string before calling join().

main.py

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my_list = ['a', 'b', 1, 2]


all_strings = list(map(str, my_list))

print(all_strings) # 👉️ ['a', 'b', '1', '2']

result = ''.join(all_strings)

print(result)  # 👉️ "ab12"

The string the method is called on is used as the separator between elements.

main.py

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my_list = ['a', 'b', 'c']

my_str = '-'.join(my_list)

print(my_str)  # 👉️ "a-b-c"

If you don't need a separator and just want to join the iterable's elements into a string, call the join() method on an empty string.

main.py

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my_list = ['a', 'b', 'c']

my_str = ''.join(my_list)

print(my_str)  # 👉️ "abc"

# Converting a list to a string before writing it to the file

Alternatively, you can pass the list to the str() class to convert it to a string before writing it to the file.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_list = ['Alice', 'Bob', 'Carl']

    my_file.write(str(my_list)) # 👉️ "['Alice', 'Bob', 'Carl']"

The str() class converts the supplied value to a string and returns the result.

You can also convert the list to a JSON string before writing it to the file.

main.py

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import json

with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_list = ['Alice', 'Bob', 'Carl']

    my_file.write(json.dumps(my_list))  # 👉️ ["Alice", "Bob", "Carl"]

The json.dumps() method converts a Python object to a JSON formatted string.

# Table of Contents

# TypeError: write() argument must be str, not dict

The Python "TypeError: write() argument must be str, not dict" occurs when we pass a dictionary to the write() method.

To solve the error, convert the dictionary to a string or access a specific key in the dictionary that has a string value.

typeerror write argument must be str not dict

Here is an example of how the error occurs.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_dict = {'name': 'Alice', 'age': 30}

    # ⛔️ TypeError: write() argument must be str, not dict
    my_file.write(my_dict)

We passed a dict object to the write() method, but the method can only be called with a string argument.

# Convert the dictionary to a string before writing it to the file

One way to solve the error is to convert the dictionary to a string.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_dict = {'name': 'Alice', 'age': 30}

    my_file.write(str(my_dict)) # 👉️ {'name': 'Alice', 'age': 30}

The str() class converts the given value to a string and returns the result.

You can also convert the dictionary to a JSON string by using the json.dumps method.

main.py

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import json

with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_dict = {'name': 'Alice', 'age': 30}

    my_file.write(json.dumps(my_dict))  # 👉️ {"name": "Alice", "age": 30}

The json.dumps() method converts a Python object to a JSON formatted string.

# Writing a specific dictionary value to the file

If you meant to access a specific key in the dictionary, use square brackets.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_dict = {'name': 'Alice', 'age': 30}

    my_file.write(my_dict['name'])  # 👉️ Alice

Make sure that the key you are accessing has a value of type string because the write() method can only get called with a string argument.

# Checking what type a variable stores

If you aren't sure what type a variable stores, use the built-in type() class.

main.py

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my_dict = {'name': 'Bobby Hadz'}
print(type(my_dict))  # 👉️ <class 'dict'>
print(isinstance(my_dict, dict))  # 👉️ True

my_str = 'hello'
print(type(my_str))  # 👉️ <class 'str'>
print(isinstance(my_str, str))  # 👉️ True

The type class returns the type of an object.

The isinstance() function returns True if the passed-in object is an instance or a subclass of the passed in class.

# Table of Contents

# TypeError: write() argument must be str, not tuple

The Python "TypeError: write() argument must be str, not tuple" occurs when we try to write a tuple object to a file using the write() method.

To solve the error, use the join() method to join the tuple into a string, e.g. my_file.write(','.join(my_tuple)).

typeerror write argument must be str not tuple

Here is an example of how the error occurs.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_tuple = ('Alice', 'Bob', 'Carl')

    # ⛔️ TypeError: write() argument must be str, not dict
    my_file.write(my_tuple)

We passed a tuple object to the write() method, but the method can only be called with a string argument.

# Join the tuple's elements into a string before writing to the file

One way to solve the error is to use the str.join() method to join the tuple into a string.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_tuple = ('Alice', 'Bob', 'Carl')

    my_file.write(','.join(my_tuple))  # 👉️ Alice,Bob,Carl

We used a comma as the separator between the items of the tuple, but you can use any other separator or an empty string if you don't need a separator between the elements.

The str.join method takes an iterable as an argument and returns a string which is the concatenation of the strings in the iterable.

Note that the method raises a TypeError if there are any non-string values in the iterable.

# Convert all values in the tuple to strings

If your tuple contains numbers or other types, convert all of the values to string before calling join().

main.py

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my_tuple = ('a', 'b', 1, 2)

all_strings = tuple(map(str, my_tuple))

print(all_strings)  # 👉️ ('a', 'b', '1', '2')

result = ''.join(all_strings)
print(result)  # 👉️ 'ab12'

The string the method is called on is used as the separator between elements.

main.py

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my_tuple = ('a', 'b', 'c')

result = '-'.join(my_tuple)

print(result)  # 👉️ 'a-b-c'

If you don't need a separator and just want to join the iterable's elements into a string, call the join() method on an empty string.

main.py

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my_tuple = ('a', 'b', 'c')

result = ''.join(my_tuple)

print(result)  # 👉️ 'abc'

# Converting the tuple to a string or a JSON string

Alternatively, you can pass the tuple to the str() class to convert it to a string before writing it to the file.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_tuple = ('Alice', 'Bob', 'Carl')

    my_file.write(str(my_tuple))  # 👉️ ('Alice', 'Bob', 'Carl')

You can also convert the tuple to a JSON string before writing it to the file.

main.py

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import json

with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_tuple = ('Alice', 'Bob', 'Carl')

    my_file.write(json.dumps(my_tuple))  # 👉️ ["Alice", "Bob", "Carl"]

The json.dumps() method converts a Python object to a JSON formatted string.

Since JSON doesn't support tuples, the tuple gets converted to a list.

# How tuples are constructedin Python

In case you declared a tuple by mistake, tuples are constructed in multiple ways:

Using a pair of parentheses () creates an empty tuple
Using a trailing comma - a, or (a,)
Separating items with commas - a, b or (a, b)
Using the tuple() constructor

If you aren't sure what type of object a variable stores, use the type() class.

main.py

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my_tuple = 'bobby', 'hadz'

print(type(my_tuple))  # 👉️ <class 'tuple'>
print(isinstance(my_tuple, tuple))  # 👉️ True

my_str = 'bobby hadz'

print(type(my_str))  # 👉️ <class 'str'>
print(isinstance(my_str, str))  # 👉️ True

The type class returns the type of an object.

The isinstance() function returns True if the passed-in object is an instance or a subclass of the passed-in class.

# TypeError: write() argument must be str, not None

The Python "TypeError: write() argument must be str, not None" occurs when we pass a None value to the write() method.

To solve the error, correct the assignment and pass a string to the write() method.

typeerror write argument must be str not none

Here is an example of how the error occurs.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_str = None

    my_file.write(my_str)

We passed a None value to the write method which caused the error.

The method takes a string and writes it to the file.

To solve the error, you have to figure out where the None value comes from and correct the assignment.

# Common sources of None values in Python

The most common sources of None values are:

Having a function that doesn't return anything (returns None implicitly).
Explicitly setting a variable to None.
Assigning a variable to the result of calling a built-in function that doesn't return anything.
Having a function that only returns a value if a certain condition is met.

# A function that doesn't return a value returns None

Functions that don't explicitly return a value return None.

main.py

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# 👇️ this function returns None
def get_str():
    print('hello world')


with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_str = get_str()
    print(my_str)  # 👉️ None

    # ⛔️ TypeError: write() argument must be str, not None
    my_file.write(my_str)

You can use a return statement to return a value from a function.

main.py

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def get_str():
    return 'hello world'


with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_str = get_str()
    print(my_str)  # 👉️ "hello world"

    my_file.write(my_str)

# Checking if the variable is not None before writing to the file

Use an if statement if you need to check whether a variable doesn't store a None value before writing it to the file.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_str = None

    if my_str is not None:
        my_file.write(my_str)

Alternatively, you can provide an empty string as a fallback.

main.py

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with open('example.txt', 'w', encoding='utf-8') as my_file:
    my_str = None

    if my_str is None:
        my_str = ''

    my_file.write(my_str)

# Having a function that returns a value only if a condition is met

Another common cause of the error is having a function that returns a value only if a condition is met.

main.py

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def get_string(a):
    if len(a) > 3:
        return a


my_string = get_string('hi')

print(my_string)  # 👉️ None

The if statement in the get_string function is only run if the passed in string has a length greater than 3.

In all other cases, the function doesn't return anything and ends up implicitly returning None.

To solve the error in this scenario, you either have to check if the function didn't return None or return a default value if the condition is not met.

main.py

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def get_string(a):
    if len(a) > 3:
        return a
    return ''


my_string = get_string('hi')

print(my_string)  # 👉️ ""

Now the function is guaranteed to return a string regardless of whether the condition is met.

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TypeError: write() argument must be str, not X in Python

# Table of Contents

# TypeError: write() argument must be str, not bytes

# Open the file in wb (write binary) mode to write bytes

# Open the file in w (write text) mode to write text

# Table of Contents

# TypeError: write() argument must be str, not list

# Use the str.join() method to join the list into a string

# All values in the collection must be strings

# Converting a list to a string before writing it to the file

# Table of Contents

# TypeError: write() argument must be str, not dict

# Convert the dictionary to a string before writing it to the file

# Writing a specific dictionary value to the file

# Checking what type a variable stores

# Table of Contents

# TypeError: write() argument must be str, not tuple

# Join the tuple's elements into a string before writing to the file

# Convert all values in the tuple to strings

# Converting the tuple to a string or a JSON string

# How tuples are constructedin Python

# TypeError: write() argument must be str, not None

# Common sources of None values in Python

# A function that doesn't return a value returns None

# Checking if the variable is not None before writing to the file

# Having a function that returns a value only if a condition is met

Borislav Hadzhiev

# Open the file in `wb` (write binary) mode to write bytes

# Open the file in `w` (write text) mode to write text

# Use the `str.join()` method to join the list into a string