Basics of Programming

Programming basics

Objectives:

Learn the 5 components of the Python Syntax
Understand Python's object system
Learn to interact with the IPython prompt in the Jupyter notebook

Jupyter, IPython, Python, oh my!

Just to keep the computational model straight, we are using:

Python 3.6, a interpreted programming language
A REPL, which Reads a line, Evaluates it, then Prints it (in a Loop)
- print not needed to see the last un-captured value in a cell
IPython, which is an enhancement to make Python more Interactive
- You can't use these enhancements in a library, but they generally are only useful interactivally anyway
Jupyter notebooks, which is a web based display of a Kernel, such as IPython (or many other language kernels).

Jupyter notebook  ->      IPython       ->      Python
-----------------    ------------------    ------------------
This is what you     Checks for special    Does the real work
see                  syntax, then sends
                     it on

Note: The very new JupyterLab takes this one step further, by providing an nice environment for notebooks, which themselves are an environment for IPython... It's really nice and beautiful, though.

Basics of programming

Computing can be broken down to two things:

Variables: These hold data
Functions: These are procedures (usually operating on variables)

# This is a variable
variable = 3

# This is a function
def function(x):
    return pow(x, 2)

function(variable)

Don't worry about the details of the Python syntax quite yet, just focus on the two concepts: data and procedures. And, yes, x**2 is nicer than pow(x,2).

Try it yourself

The great thing about programming in an interpreted language like Python is that you can try everything yourself, in real time. Play around with the above variables and answer these questions:

Can you name a variable def? Why or why not?
Can you name a variable pow? Why or why not?
Why might you not want to make a variable named pow?
How can you get rid of your variable named pow? (Hint: Try del or restarting your kernel (top menu)).

Python's syntax

Operators: special symbols or words that have a specific meaning. You can usually control what they do in classes. Some general parts of the syntax, like whitespace, commas, colons, and brackets, are not called operators.
Keywords: special words that have language level meaning and cannot be used elsewhere or changed.
Builtins: functions and objects that are pre-defined, but not special.
Numbers: several different number types available
Strings: add text to the program or help
Comments: start with a # and are ignored by Python

Note: Builtins can't be undefined due to the way the Python lookup system works.

# Does not take anything or return anything
def funct():
    "This is the function documentation"
    print('Called f', 1*1, 'time')

Operators, Keywords, Builtins, Numbers, Strings, Comments

Answer this:

What color is each of these in your notebook?

Objects and Types/Classes

We can go one step further from variables and functions: objects and classes.

Object: A related collection of data and functions for that data
- Members: Variables that hold the data for the object
- Methods: Functions that operate on the object data
Class: The "type" of an object; objects that have the same methods/members have the same class

Why are objects so important in Python? Because unlike many other languages, everything in Python is an object! This is very powerful.

Note: Syntax such as keywords are not objects. (though builtins are)

number = 1.5
string = "hello world"
complex_number = 2 + 3j

Try running some of the methods and look at the members of the objects defined above.

IPython + Jupyter keyboard hints:

Use Tab to see completions, either when typing a word or after a .
Use Shift-Tab to see the help for a function after typing it
Use Shift-Enter to execute a cell

We can use type to see the class of an object:

type(number)

Names

There are certain conventions for the names of objects and classes:

Case	Convention	Example
Objects and methods	start with lower case letter	`like_this`
Classes	Start with upper case letter	`LikeThis`
Globals	all uppercase	`LIKE_THIS`
Hidden	start with underscore	`_like_this`
In one class only	start with two underscores (rarely used)	`__like_this`
Special python names	start and end with two underscores	`__like_this__`

Exceptions for some Python built-in types, which are all lowercase. And True, False, None, and Ellipsis, which are objects but are capitalized.

Built in Python Types

The following are a mostly comprehensive list of built-in Python types

Basic types

py_int = 101
py_float = 3.7
py_complex = 1j
py_str = "Unicode string: Python 3 is 😀"
py_bytes = b"Binary string"
py_bool = True
py_none = None

These types are all also constructible by calling the name of the type, such as int(1). A few interesting notes:

Numbers can be constructed from strings too, like int("1")
Strings can start with special characters, like b. We'll see this again someday.
Strings use " or '. Three quotes can be used to include new lines.
None is the only none type. How lonely.

Try it yourself:

How large is the largest int value allowed?

Compond types

These are composed, but are also very important

py_tuple = (1,2,3)
py_list = [1,2,3]

py_dict = {"Python 3":"😍",
           "Python 2":"😖"}

py_set = {"no", "duplicates", "in", "here"}

Accessing values is done with brackets:

print(py_tuple[0])
print(py_dict['Python 3'])

Indexing starts from 0, as it should
A tuple is just like a list, but can't be changed: immutable

Mutable	Immutable
Can be changed	Can only be replaced
Can be a key or in a set	Can't be a key or in a set
`list`, `dict`, `set`	`tuple`, `str`, `int`, etc.

A set does not have an order (so you can't use [] to get items from it)
Dictionaries are ordered before Python 3.6 (even now you still only have direct access to keys)

There are other types in the standard library, but we'll restrict ourselves to talking about these for now.

If you could put a mutable item in as a key or set item, you could then change it later and have two identical key items or set items!

Try it out: Mutation vs. replacement

Can you guess what the final value of a is? What about b?

a = [1,2,3,4]
b = a
b[2] = "😡"
b = [3,2,1,3]
b[2] = None

print(a,b)

Key points to understanding this example:

Normal assignment replaces the variable on the left
Python never copies, but with immutable values and the above statement, it might seem that way

There are a limited number of expressions allowed on the left (but they can be chained):

An object (replaces): a =
An index into an object (mutate) a[x] =
An object attribute (mutate): a.x =

Slicing

One of the most important things in computation is indexing, and Python is really good at it. You've already seen integers being used to index lists (strings work the same way too), but let's peek at another way to index lists and tuples: slicing.

a = "abcdefg"
print(a)
print(a[0:7:1]) # start : stop (not included) : step
print(a[0:7])   # step not needed
print(a[1:])    # you can leave out values
print(a[:-1])   # like indexing, - means from end
print(a[::2])

Operators

Python's operators are pretty standard. Here are the ones that tend to be a little odd:

x**y: Raise x to a power y
x == y: See if x is equal to y
x is y: See if x is the same object (in the same spot in memory) as y, can't be overriden
~x: Invert x (might be different than unary -)
+x: Python does have a unary + operator, too. Not sure why you'd want it.
x += y: Add y to x, in place only if mutable

x / y: True mathematical division (Py3 default, Py2 option)
x // y: Truncating division
x @ y: Matrix product of x and y (Python 3.5+, only in libraries)
x and y: Boolean operator, short-circuits and can't be overriden (or and not too)
x & y: Bitwise and - this one can be overriden, but has odd precedence
x | y: Bitwise or

Control

We've already seen functions. What other control statements are out there for use to use?

x = True
if x:
    print("x was true")
else:
    print("x was not true")

The for each loop works on any iterable:

a = "abcdefg"
for item in a:
    print(item, sep=', ')

You can also use for item in range(N) to count over a range of values, or while CONDITION:.

You can use for (and if) inside [], (), or {} to build the data structures mentioned above inplace.

bad_way_to_make_a_set_of_tens = {x for x in range(100) if x % 10 == 0}
bad_way_to_make_a_set_of_tens

With

There are other useful things in Python we will cover later, but the with statement is too good to wait. With lets you take code like this:

f = open(filename) 
txt = f.read()
f.close() # Don't forget me!

and write instead:

with open(filename) as f:
    txt = f.read()
# File automatically closed here!

With simply runs code at the start and at the end of a block. It also promises the code at the end runs, even if there's an error! (More about errors later)

Isn't all of this restrictive?

You may have noticed that each new concept in programming restricts rather than enables. If we went further into topics like functional program, this would continue to be the case. This is odd but true: we could write any program with a very small set of constructs. However, the most important feature of programming is organization (also called design).

Compare this hypothetical pseudo-code:

i = 0
label start
compute(i)
i = i + 1
if i < 10: goto start

Verses real code:

for i in range(10):
    compute(i)

Now imagine a complex program with 1,000 of goto statements; you would have to work through every line of code to understand it. But if you restrict yourself to common structures, like loops, objects, functions, etc., you no longer have to look at the whole program, but just parts you recognize.

Design in programming

You should:

Make small, understandable pieces that can be run by themselves (Easier to debug and test)
Write similar code once (fewer places for bugs)
Reuse existing functions or libraries when possible (let someone else design and debug what they are good at)

Objects are very good for the second two points (and so-so for the first).

Also notice the running theme above?