write() argument must be str not float

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

Last updated: Jan 29, 2023 Reading time · 11 min

# Table of Contents

• TypeError: write() argument must be str, not BYTES
• TypeError: write() argument must be str, not LIST
• TypeError: write() argument must be str, not DICT
• TypeError: write() argument must be str, not TUPLE
• TypeError: write() argument must be str, not NONE

# 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.

Here is an example of how the error occurs.

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.

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.

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.

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.

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

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.

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

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

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 .

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 .

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

# 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.

# 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.

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

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.

# 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() .

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

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.

# 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.

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.

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

# 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.

# 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.

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.

# Writing a specific dictionary value to the file

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

# Checking what type a variable stores

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

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 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)) .

# 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.

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.

# 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() .

# 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.

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

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.

# 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.

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

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

# 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 .

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

# 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.

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

# 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.

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

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.

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

Borislav Hadzhiev

Web Developer

Copyright © 2024 Borislav Hadzhiev

must be str, not float

Answered on: Sunday 30 July, 2023 / Duration: 12 min read

Programming Language: Python , Popularity : 4/10

Solution 1:

In Python, the error "must be str, not float" occurs when you try to concatenate a string with a float using the "+" operator. Python is a strongly-typed language, which means it requires explicit conversion between data types. When you try to concatenate a string with a float directly, Python raises a TypeError because it cannot implicitly convert the float to a string.

Let's look at some code examples to illustrate this error and how to resolve it:

Example 1: Concatenating a string with a float (causing an error)

In the example above, Python raises a TypeError because we attempted to concatenate the string literals with the integer age and the float height without converting them to strings explicitly.

To fix this error, we need to convert the integer and float to strings before concatenation using the str() function.

Example 2: Fixing the error by converting float and int to string

In this corrected example, we explicitly converted the integer age and the float height to strings using str() before concatenating them with the rest of the string. Now the program runs without any errors, and the output is as expected.

Remember, when working with different data types in Python, always ensure to perform explicit conversions between them when needed to avoid errors like "must be str, not float."

Solution 2:

In Python, when you use the str() function to convert a number to a string, it will automatically choose the appropriate type of string based on the value of the number. For example:

This will print the string '42' because the value of x is an integer. However, if the value of x is a floating point number, the str() function will create a string representation of the number using scientific notation by default. For example:

This can be unexpected and may not be what you want. To avoid this behavior, you can specify the formatting of the string using the format() method. For example:

Alternatively, you can use the repr() function which returns a string containing the representation of the object. It is similar to str() , but it doesn’t call the __str__() method on the object. Instead, it calls the __repr__() method. The difference between __str__ and __repr__ is that __str__ gives more control over how objects are represented as strings, whereas __repr__ aims to provide a more informative representation that can be used for debugging purposes.

As you can see, both str() and repr() functions produce the same output for the custom class MyClass . But the advantage of using repr() is that it produces the same format for all types of inputs whereas str() might produce different formats depending on the input.

In summary, if you want to convert a number to a string in Python without scientific notation, you can use the format() method or the repr() function. The choice between them depends on your specific use case and personal preference.

Solution 3:

The error "must be str, not float" in Python usually occurs when you are trying to concatenate a string object with a float object.

For instance, see the case below:

The output of the above will result in a TypeError:

In Python, a string cannot be directly merged with a float or integer. The types are different: one is a string (text), while the other is a float (a decimal number). In order to merge a number with a string, you must convert the number to string format first.

See the corrected code:

The output will be:

By using the str() function, we are converting the float age to a string that can be then concatenated with the other string.

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