Introduction to Python and its Implementations

This  is in continuation of Programming in C-Python . It provides information about different Python implementations., built-in functions. Identifying Python version and how it finds modules

What are various Python Implementations ?

As we know there are various C implementations such as ANSI C,C95,C99 etc.

Similarly there are various Python implementations:

• CPython : Written in C. This is the original Python.
• Jython: Written in Java
• PyPy Written completely in Python. Supports advance features such as stackless support, JIT(Just In Time) compiler
• IronPython Written in Python for for  .NET applications

Which Python is installed in my setup ?

>>> import platform

>>> platform.python_implementation()‘CPython’

Where is Python installation folder ?

>>> import sys

>>> sys.executable‘C:\\Users\\shreyash\\AppData\\Local\\Programs\\Python\\Python36\\python.exe’

One can find this in Windows cmd line with echo %PATH%

Comments

In C, Line comment start with // while in Python it is with #

Each line in C requires a well defined statement followed by termination (;) .

Each line in Python can have any assignment,expression,function or method without any termination symbol . It can have simple calculator kind of expression e.g. 1+2

Multiple Lines

In C, we use \ to extend macro definition to multiple lines

#define TEST()\
line1\
line2

In Python, we can use \ to make a multiple physical lines(CR LF) statement to a single logical statement.

>>> if rollno > 1 and rollno <20 \
... and marks >50 :\
...        print(marks)

This is not required for statements written in parenthesis, square or curly brackets

Grouping of Statements

C & C++ uses braces { }  to define a block of code or grouping statements .

Python uses leading white spaces or tabs in logical line to start the indentation and end with an empty logical line.

>>> count=10
>>> if count>100:
...    print(count)
...
>>> print(count)
10
>>>

Interactive Python

You may try ipython  or simply use a text file e.g xyz.py

Interactive Python can be used for quick testing of some expression, but whatever command you enter will be lost after experiment. So you need to use a text editor such as notepad/vi to create .py file similar to .c

For ex: python test.py  #To execute test.py file

One can use -i option to analyze the data after test.py executed without terminating it. This is very useful for debugging purpose.

python -i test.py  #Good for debugging purpose

Module Integration

In C/C++ header file is used to access modules/functions in other source files/library. For ex : #include<stdio.h> in C for accessing printf()

In Python, import statement is used to access  modules if it is not part of builtin functions.  modules command in help> can provide list of available modules

What builtin functions are available ?

>>> import builtins
>>> dir(builtins)
['ArithmeticError', 'AssertionError', 'AttributeError', 'BaseException', :::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::::'int', 'isinstance', 'issubclass', 'iter', 'len', 'license', 'list', 'locals', 'map', 'max', 'memoryview', 'min', 'next', 'object', 'oct', 'open', 'ord', 'pow', 'print', 'property', 'quit', 'range', 'repr', 'reversed', 'round', 'set', 'setattr',
::::::::::::::::::::::::::::::::::::

Using factorial and sqrt functions from math module.

C:\Users\shreyash>python
Python 3.6.6 (v3.6.6:4cf1f54eb7, Jun 27 2018, 03:37:03) [MSC v.1900 64 bit (AMD64)] on win32
Type "help", "copyright", "credits" or "license" for more information.
>>> import math
>>> math.factorial(4)
24
>>> math.sqrt(16)
4.0
>>>

Functions inside module can be found using dir

>>>dir(math)

How Python finds  modules ?

PYTHONPATH environment variable can also be used to define module search path. The path utilized to search modules can be found using

>>> import sys

>>> sys.path

References 

• History of C language
• Python Documentation
• Python Documentation  C:\Users\shreyash\AppData\Local\Programs\Python\Python36\Doc

I have discussed this at  Programming C-Python video.

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