Python Interpreter

 Python interpreter is a translator. 

 ➢ Python compiler generates bytecode for the interpreter. 

 ➢ The python interpreter uses the virtual machine. 

 ➢ The same bytecode doesn’t always make python dynamic. 

➢ The default prompt for the interpreter is >>>. 

➢ The default encoding python source file is UTF-8 (Unicode Transformation Format) . 

 ➢ Unicode standard variable-width character encoding can encode 1,112,064 valid code points using up to four 8-bit bytes. 

 ➢ It uses characters of most languages as strings, literals, comments, and identifiers. 

 ➢ Its standard library uses ASCII characters only.

➢ We add a comment, Windows-1252 encoding as in the file as - 

➢ # -*- coding: encoding -*- 

➢ # -*- coding: cp1252 -*- 

➢ When you want to begin code with a UNIX shebang line:- 

➢ #!/usr/bin/env python3 

➢ # -*- coding: cp1252 -*- 

➢ In the window, the interpreter can find an address like C:\Python36 

➢ In Unix /usr/local/bin 

➢ We can set the path at the time of installation or using this command: 

set path=%path%;C:\python36 

On Windows, to run the Python interpreter in the shell, type: $python 

➢ To get out of the interpreter in disassembling the Bytecode shell, type: >>> quit() 

➢ Alternately, you can use an end-of-file character at the prompt. 

 ➢ Python interpreter exits with a zero exit status. 

 ➢ You can use it in a REPL (Read-Evaluate-Print-Loop) fashion. (Read: take user input.; Eval: evaluate the input.; Print: shows the output to the user.; Loop: repeat.) 

➢ if you want, you can save your Python code as a script and execute then using: $python demo.py 

➢ To enter interactive mode after running a script, you can pass –i before the script. The command python -c command [arg] … executes statements in command, and the python - m module [arg] … executes the source file for the module. 

➢ All command-line options are described in the Command line and environment. 

Features of Python Interpreter 

➢ Python interpreter offers some features: 

❖ Interactive editing 

❖ History substitution 

❖ Code completion on systems with support for reading line 

 ➢ In the first Python prompt, pressing the Ctrl+P:- Tells you if your interpreter supports command-line editing. 

 ➢ A beep indicates that it does support command-line editing. 

 ➢ Otherwise, it will either perform a no-operation or echo ^p to indicate it isn’t available.

Passing Arguments 

➢ You pass a script name and additional arguments to the shell. 

 ➢ Python interpreter turns these into a list of strings and assigns these to the variable argv in the sys module. 

 ➢ The following command will give us a list of thisimport sys 

➢ Without a script or arguments, sys.argv [0] denotes an empty string. 

 ➢ A script name of ‘-‘means that it sets sys.argv [0] to ‘-‘, and with ‘-c’, it is set to ‘-c’. 

 ➢ A value of ’-m’ sets sys.argv [0] to the module’s full name. 

 ➢ The command (‘-c’)/ module(‘-m’.) handles the options. 

Interactive Mode 

➢ In Python interpreter, interactive mode reads commands from a tty (teletype:- terminal control function). 

 ➢ The primary prompt (first ) is the following: >>> 

➢ It shows prompts the developer for the next command. 

 ➢ This is the A read–eval–print loop (REPL). 

 ➢ Python interpreter prints a welcome message, version number, and a copyright notice Before it prints the first prompt. $ python3.8Python 3.8 (default, Sep 16 2015, 09:25:04)[GCC 4.8.2] 

on Linux type "help", "copyright", "credits" or "license" for more information.>>> 

➢ The secondary prompt (Continuation lines) is: … 

➢ Continuation lines are needed when entering a multi-line construct. 

example:- if statement: >>> if the_world_is_flat = True: ... ("Be careful not to fall off!")

Python Interpreter Working:- 

➢ Four things in a REPL: 

I. Lexing- The lexer breaks the line of code into tokens. 

II. Parsing- The parser uses tokens to generate a structure, as, an Abstract Syntax Tree, to depict the relationship between these tokens. 

III. Compiling- The compiler turns this AST (abstract syntax tree) module to help Python applications process trees of the Python abstract syntax grammar. An abstract syntax tree can be compiled into a Python code object using the built-in compile () function. 

IV. Interpreting- The interpreter executes each code object. These processes divide into main following parts :- 

a. Function Objects & Code Objects 

b. The Bytecode 

c. Dis-assembling the Bytecode 

a. Function Objects & Code Objects 

➢ In Python, functions are first-class objects. You can pass them without making a call to them. 

>>> def bar(a): x=3 return x+a >>> bar 

b. The Bytecode 

➢ This is a series of bytes, each of which the interpreter loops through and then makes an execution. Ex:- >>> bar ‘d\x01\x00}\x01\x00|\x01\x00|\x00\x00\x17S’ 

➢ >>>import dis 

➢ >>> dis.dis(bar) 

c. Dis-assembling the Bytecode 

➢ The dis module supports the analysis of CPython bytecode by disassembling it. 

 ➢ The CPython bytecode taken as an input is defined in the file Include/opcode.h and used by the compiler and the interpreter. 

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