Error Encyclopedia | Type Error

Types in Python

A type in Python can be thought of as the specification for a category of related data. Though we as programmers can technically define own types, they tend to fall into a set of pre-defined categories. So, for now, let's focus on the most common types that are built into the Python language. Specifically, they are:

int -- whole numbers
float -- decimal numbers
str -- a sequence of characters, like the text of this page
list -- a list of things in a given order
tuple -- an immutable list of things in a given order
boolean -- a truth value; either True or False

For further clarification, here are some pieces of data and their associated types:

        
          >>> 25 # an int 

          >>> 3.14 # a float 

          >>> "Neigh!" # a string 

          >>> [2, 13, 10] # a list (containing three objects of type int) 

          >>> (5, 0, 7) # a tuple --- much like a list, but it can't ever be modified

When you call a function or use an operator in Python, it typically expects to be given parameters of a specific type. For example, the multiplication operator (*) expects to be given two parameters of numeric type, that is, the types int and float:

        
          >>> 49401314 * 150 

          7410197110 

          >>> 1187228.3001583007 * 7 

          8310598.101108105

Type Errors in Python

A TypeError occurs in Python when you attempt to call a function or use an operator on something of the incorrect type. For example, let's see what happens when we try and add together two incompatible types:

      	
	        >>> 2 + "two" 

            Traceback (most recent call last): 

              File "<stdin>", line 1, in <module> 

            TypeError: unsupported operand type(s) for +: 'int' and 'str'

Because the plus operator (+) expected two numeric parameters, Python throws a TypeError, telling us that one of our parameters was of the incorrect type. Let's take a look at another example:

        
          >>> myVar = 30 

          >>> myVar[1] 

          Traceback (most recent call last): 

            File "<stdin>", line 1, in <module> 

          TypeError: 'int' object is not subscriptable

Because we can't access int type objects by index, as we can with list, tuple, or string type objects, Python once again throws a TypeError. In PyLearn, the message would be displayed as follows:

        
          >>> myVar[1] 

          TypeError: Can't access an index for an object of type -- 'int'.

In general, if you've encountered a TypeError in Python, take a moment to verify that the data you are operating on is actually of the type you expected it to be. To do so, simply call the built-in type() function on the questionable data, and Python will gladly report its type to you:

        
          >>> type("lovely") 

          <type 'str'>