All values have a type and an identity. Built-in functions type() and id() return respectively the type of a value and its identity. The identity of a variable is a reference to where its value is stored in computer memory.

The simple value (primitive) types are int, float, and bool. Primitive type values have no behaviors.

The values (objects) of types such as str and list do have behaviors. Behaviors are implemented as methods (message functions). A method function invocation tells its object to carry out a behavior. For example, if s is a string, then s.lower() returns a new copy of s where are letters in s are in lower case form in the copy.

Literal values are composed of explicit strings, numbers, and logical values like 3, 0.1415, True, [ 2, "abc", 5.85 ].

Operators like +, /, //, %, and ** evaluate their operands and return the value of the corresponding mathematical operation save Python limitations on decimal representations.

`str`

Once created str values are immutable character sequences (there is no function, method, or operator that can modify an existing string).

Escape sequences, like "\n" represent the new-line character.

Operators [ i ] and [ i : j ] can retrieve substrings.

Operator + produces a new sring formed by concatenating its operands.

Built-in function len() to find how many characters there are

Methods like find() and upper() return values.

`range`

range( i , j ) is the sequence of integer values from i to j-1.

`list`

Literal values like [] and [ 1, "two", [ 3, 3.0 ] ].

Operators [ i ] and [ i : j ] can retrieve or modify elements or sublists

Operator + with list operands produces a new list by concatenating the elements of its list operands.

Operator + cannot be used when only one element is a list.

Operatop in checks if a value is within the list can be a range or a list.

Functions like len(), max(), and sum() to analyze the contents of a list.

Method functions like index() return values.

Method function append() modifies the list in place (and returns None).

For a lot more discussion go here.

`for` loops

Repeat statements by iterating over a sequence of values. The sequence is a typically list or a range, but can also be a string.

for item in sequence :
statement₁
statement₂
...
statement_n

In the above, iterator variable item takes on each of the values of sequence in turn. For each of those values, statement₁, statement₂, ..., and statement_n are repeated.

A for loop inside another for loop is called a nested loop.

for loops can be used to build up (accumulate) a result.

If the result is to be a sum, then the accumulator is initialized before the loop to 0 and increased within the loop using numeric operator +.

If the result is to be a string, then the accumulator is initialized before the loop to "" and added to within the loop using string operator +. Observe that "" is a string of length 0 and different from the string " " which is a string of length 1 that consists of a single splace.

If the result is a list, then the accumulator is initialized before the loop with [] and added to within the loop using append().

If the result is to be a product, then the accumulator is initialized before the loop with 1 and increased within the loop using numeric operator *.

Producing values

Values are produced whenever Python evaluates an expression.

Anytime an expression is encountered it is evaluated.

Anything following an assignment operator is an expression.

Any argument to a function or method is an expression.

The operand(s) for an operator are expression(s).

An expression can be composed from literals, variables, operators, and sub-expressions.

Functions and methods

A built-in function invocation does not have an object to which its actions are targeted; e.g., max( a, b ) returns the larger of values a and b.

A method is named piece of code that manipulates the object to which it is targeted (dotted); e.g., in invocation s.capitalize() method capitalize() examines s to produce and return a copy of string s with the first character capitalized and the other characters in lowercase.

Whenever a function or method is invoked it, returns a value, even if that value is just None (like it is for print() and a.append()).

Library

A library is a modue of related functions and values that must be imported to gain access to them.

Standard library math provided access to a wide variety of mathematical computations and values; e.g., math.sqrt(), math.sin(), and math.pi.

Local library url provides web access

url.get_contents( link ) returns the contents of the web resource indicated by web location link.

url.get_dataset( link ) returns the contents of the web resource indicated by web location link as a dataset sheet whose cell values are either string, int, float, or bool as appropriate.

Web processing

Suggest redownloading local module url.py.

Use url.get_contents() to get the contents of a web file.

Use url.get_dataset() to get the contents of a web dataset.

Errors

Syntax errors are illegally formed Python instructions. This web page epistle might prove useful in dealing with syntax errors.

Semantic errors are actions not doing what is intended.

Gotchas

Ignoring the Bill Murray's Rule of Life — baby steps to obtain your goals.

Ignoring the Fernando Lamas' Rule of Life — looking good makes a life complete.

Consistent use of whitespace around operators and blank lines make mindfulness easier to obtain.

Forgetting the Law of Cole from grade school or even pre-grade school.

Not reading the complete description before starting to problem solve

Ignoring the input order

Not parsing (converting) the input into the needed values

Not converting a string to a number using int() or float() or to a Boolean value using bool().

Output expectations

Improper accumulation

Combining a string and a number using the + operator

Using only the first or last value of a range or sequence to compute result

Not checking whether your solutions match given test run expectations

Not mindfully writing and reading your solution — you must understand what you are writing

Mindfullness makes your computational journey clearer and easier

Corollary: use print() statements when developing code to document computational progress and comment out those statement before submitting

Functions use parentheses; lists use brackets. Remember this.

Functions must always have parentheses. Remember this.

https://en.m.wikipedia.org/wiki/File:Bavarian_Krautsalat_Munich.JPG

Class 14 — Friday September 24

Knowing what you know

Look both ways

Agenda

Examples

Recent Test 1 solutions

Review module documentation

Epistles

Chrestomathics

Development and translation

How Python works

Values and types

str

range

list

for loops

Producing values

Functions and methods

Library

Web processing

Errors

Gotchas

`str`

`range`

`list`

`for` loops