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Part 4: Mixing Type is (Sometimes) an Error

Part 4: Mixing Type is (Sometimes) an Error#

Python does not allow us to add strings and numbers together, only pairs of numbers or pairs of strings:

>>> "abc" + 123
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "int") to str
>>> 123 + "abc"
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: unsupported operand type(s) for +: 'int' and 'str'

Read these error messages carefully. They use a fair amount of vocabulary we’ve introduced, like concatenate, operand, and type. Python refers to the type of integers as int and the type of strings as str in its error messages.

Similarly, you cannot use the string replication operator with both the operands as strings.

>>> 'CS ' * '515-A'
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can't multiply sequence by non-int of type 'str'

The above error message says that we cannot multiply a string (which is a sequence of characters) by a string (which is not an integer).

Functions with Numbers and Strings#

There are also some useful functions that work with integers and strings. For example, the function int:

int(s)

Takes a string s made up of numeric characters and produces an integer. Reports an error if s is not a string made up of digits.

For example:

>>> int("123")
123
>>> int("this is just text")
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
ValueError: invalid literal for int() with base 10: 'this is just text'

You may expect that there’d be a related function that takes an integer and turns it into a string, and you’d be right! The function str does this:

str(v)

Takes a value v and produces its string representation.

For example:

>>> str(123)
'123'

Note that 123 and ‘123’ are not the same value! The first is an integer and can be used in arithmetic, and the second is a string and can only be used in string operations. For example:

>>> "abc" + str(123)
'abc123'
>>> "abc" + 123
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
TypeError: can only concatenate str (not "int") to str
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