1.20 Itse1359-1080-lists part 1  (Page 3/6)

The character type

Unlike many other programming languages, there is no character type in Python. Rather, a string's items arecharacters. A character is not a separate data type but is a string of exactly one character.

A sequence

The Python Language Reference -- 6.3.2. Subscriptions indicates that:

Python provides three kinds of sequences: strings , lists , and tuples .

The Python Standard Library -- 4.6. Sequence Types -- list, tuple, range states:

"There are three basic sequence types: lists, tuples, and range objects. Additional sequence types tailored for processing of binary data and text strings are described in dedicated sections."

Further down the page, the Python Standard Library -- 4.7. Text Sequence Type -- str states:

"Textual data in Python is handled with str objects, or strings. Strings are immutable sequences of Unicode code points."

From that, we can conclude that at least the following are sequence types in Python:

• lists
• tuples
• range objects
• strings

I discussed strings in previous modules. I will discuss lists in this module, and I will discuss tuples and range objects in future modules.

A mapping

The Python Standard Library - 4. Built-in Types states:

"The principal built-in types are numerics, sequences, mappings, classes, instances and exceptions."

A mapping is a structure in which values are stored and retrieved according to a key . This is often called a dictionary, because it behaves similarly to a common dictionary.

The Python Standard Library -- 4.10. Mapping Types -- dict states:

"A mapping object maps hashable values to arbitrary objects. Mappings are mutable objects. There is currently only one standard mapping type, the dictionary."

A real dictionary example

For example, if I want to know about the word Python, I get out my Webster's Seventh New Collegiate Dictionary, (which is at least 55 years old) , and I look up the word python . In this case, python is the key.

The (approximate) value associated with the key is, "... monstrous serpent killed by Apollo ..."

Is this a definition or a value ?

In normal conversation, we frequently refer to the value obtained from a common printed dictionary as the definition of the word. However, definition usually means something completely different in computer jargon.

In computer jargon, we refer to it as the value associated with the word ( key ). Thus, a mapping , is a structure that associates values with keys .

So, a common printed dictionary is an example of a mapping.

No mention of the Python language

Since my dictionary is much older than the Python programming language, (and probably older than the author of Python) I wouldn't expect to find anything about Python programming there.

What about the expression_list ?

According to the Python Language Reference -- 6.3.2. Subscriptions ,

"If the primary is a mapping, the expression list must evaluate to an object whose value is one of the keys of the mapping, and the subscription selects the value in the mapping that corresponds to that key."

In other words, the expression_list must evaluate to an object whose value is one of the keys of themapping (such as the word python in my common dictionary example) .