Introduction to Python

CIPPUS_Python   1st week

Yuanzheng Ci, Screw Studio @ DUT,

2017-11-5

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Python Philosophy

Python Philosophy

The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

Python Philosophy

Here is the plan: when someone uses a feature you don't understand, simply shoot them.

This is easier than learning something new, and too long the only living coders will be writing in an easily understood, tiny subset of Python 0.9.6

---Tim Peters

Python:

Dynamic&Strong Typing

Python:

Dynamic&Strong Typing

Interpreter

Develop Environment

• Python Shell
• Jupyter Notebook
• Ipython
• ptpython
• ...

Distribution

REPL

Read-Eval-Print Loop

Python2? Python3?

• <> to !=
• 5/2 = 2 to 5/2 = 2.5 & 5//2 = 2
• print statement to print() function
• unicode str u“str”  as default
• unicode variable name allowed
• new features...

What's in there?

What's in there?

Anatomy of a Call Expression

1. Evaluate the operator and then the operand subexpressions
2. Apply the function that is the value of the operator subexpression to the arguments that are the values of the operand subexpression

Evaluating Nested Expressions

Question

What is the value of the final expression in this sequence?

NameSpace

Assignment Statements

1. Evaluate all expressions to the right of = from left to right.
2. Bind all names to the left of = to those resulting values in the current frame.

Solution

Defining Functions

1. Create a function with signature <name>(<formal parameters>)
2. Set the body of that function to be everything indented after the first line
3. Bind <name> to that function in the current frame

Calling User-Defined Functions

1. Add a local frame, forming a new environment
2. Bind the function's formal parameters to its arguments in that frame
3. Execute the body of the function in that new environment

NameSpace/Environment

Every expression is evaluated in the context of an environment.

Life Cycle of a User-Defined Function

Def statement:

A new function is created!

Name bound to that function
in the current frame

Calling User-Defined Functions

1. Add a local frame, forming a new environment
2. Copy the parent of the function to the local frame: [parent=<label>]
3. Bind the function's formal parameters to its arguments in local frame
4. Execute the body of the function in that new environment

Environment for Nested Def Statements

Environment for Nested Def Statements

Lambda Expressions

Lambda Expressions

square = lambda x: x * x

def square(x):
    return x * x

Both create a function with the same behavior.

Conditional Statements

While Statements

For Statement Execution Procedure

Question

And