About "Not all arguments converted during string formatting" error in Python

✋ Update: This post was originally published on my blog decodingweb.dev, where you can read the latest version for a 💯 user experience. ~reza

TypEerror: not all arguments converted during string formatting in Python occurs for the following reasons:

1. When you’re formatting a string and number of placeholders and values don’t match
2. When calculating a division’s remainder and the lefthand-side operand is a string

Here’s what the error looks like:

Traceback (most recent call last):
  File test.py, line 4, in <module>
    print('Hi %s' % (first_name, last_name))
TypeError: not all arguments converted during string formatting

When you're formatting a string

As you probably know, Python supports multiple ways of formatting strings (a.k.a string interpolation):

1. Old string formatting with %
2. str.format()
3. Formatted string literals (a.k.a f-strings)

Old string formatting with the %

In the old string formatting (a.k.a printf-style string formatting), we use the % (modulo) operator to generate dynamic strings:

string % values

The string operand is a string literal containing one or more placeholders identified with %. The values operand can be a single value or a tuple of values. These values replace the placeholders in the string (in the order they're defined)

Here's an example:

first_name = 'Tom'
last_name = 'Jones'

print('Hi dear %s' % first_name)
print('Full name: %s %s' % (first_name, last_name))

And the output would be:

Hi dear Tom
Full name: Tom Jones

The s following the % is a format specifier, which in this case, specifies the replacement value is of type string. However, if you're inserting a numeric value, you can use %d instead.

✋ Watch out for: The error "TypeError: not all arguments converted during string formatting" occurs when you want to format a string, but the syntax is invalid - usually when the number of placeholders and values don't match up.

For instance, when you supply more values than the existing placeholders in your format string syntax:

first_name = 'Tom'
last_name = 'Jones'

print('Hi %s' % (first_name, last_name))

# ⛔ Causes TypeError: not all arguments converted during string formatting

You'll even get this error if you have supplied values, but there's no placeholder in the string:

first_name = 'Tom'
last_name = 'Jones'

print('Hi' % first_name)

# ⛔ Causes TypeError: not all arguments converted during string formatting

So if you're using the old-style string formatting, ensure the number of placeholders and values match.

Format String Syntax (str.format())

A more robust way of formatting strings is using the str.format() method.

In Format String Syntax, you define a string containing a literal text and a set of replacement fields (surrounded by curly braces). Then, you call format() to provide values for those replacement fields.

first_name = 'Tom'
last_name = 'Jones'

print('Hello, {0} {1}').format(first_name, last_name)
# Output: Hello, Tom Jones

When passing positional arguments to the format() method, you can omit positional argument specifiers, like so:

first_name = 'Tom'
last_name = 'Jones'

print('Hello, {} {}').format(first_name, last_name)
# Output: Hello, Tom Jones

Additionally, you can use keyword arguments with the format() method:

first_name = 'Tom'
last_name = 'Jones'

print('Hello, {fname} {lname}').format(fname=first_name, lname=last_name)
# Output: Hello, Tom Jones

Formatted string literals

You can also use Formatted string literals (also called f-strings for short) as an alternative to str.format(). Formatted string literals let you include the value of Python expressions inside a string. You create an f-string by prefixing it with f or F and writing expressions inside curly braces.

first_name = 'Tom'
last_name = 'Jones'

print(f'Hello, {first_name} {last_name}')
# Output: Hello, Tom Jones

F-strings are what you might want to use for string interpolation.

✋ Watch out for: Based on some threads on StackOverflow, some users had mixed up the old formatting syntax with the new string formatter. For instance, they used curly braces in the old string formatting syntax:

first_name = 'Tom'
last_name = 'Jones'

# Raises the error
print('Hello {} {}' % (first_name, last_name))

You must use the modulo (%) operator when using the prinf-style string formatting.

When calculating a division's remainder

You might get this type error when using the % (modulo) operator to get the remainder of a division if the lefthand operator is a string.

As you probably know, the % (modulo) operator is used for two purposes in Python:

1. Old string formatting: string % values
2. Getting a division's remainder: 12 % 2

When the % follows a string, Python considers the expression to be string formatting syntax - rather than an expression calculating a division's remainder.

The following code is supposed to print "n is Even!" if the n is an even number.

n = '12'

if (n % 2 == 0):
    print('n is Even!')

# ⛔ Causes TypeError: not all arguments converted during string formatting

However, it raises TypeError: not all arguments converted during string formatting. The reason is Python's interpreter considered it to be an interpolation syntax.

That said, if you got the number via the input() function, you have to cast it to a number first:

Problem solved!

Alright, I think it does it! I hope you found this guide helpful.

Thanks for reading.

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