Python Interpreter & Running a Python File

Python is an interpreted programming language, allowing a programmer to write and execute a python file or module directly, without any requirement to compile the program

Python (.py) files are written and then passed into the python interpreter to be executed

What’s the Python Interpreter

The interpreter is the program required to run Python code and scripts

The python command (which is a reference to the python executable that comes with a Python installation) invokes the interpreter

Technically, the interpreter is a layer of software that works between a program and computer hardware to make the program run

In the core implementation of the language, interpreter is a program written in C, like CPython

The interpreter is able to run Python code in two different ways:

• As a script or module
• As a piece of code typed into an interactive session

Python Interpreter interactive session

Python code can be run through an interactive session

To start a Python interactive session, open a command-line or terminal and then type

python

or for Python 3 type

python3

and then press Enter

Interactive session prompt (>>>)

Typically, by mentioning interpreter people refer to running interpreter in interactive session

The standard prompt for the interactive mode is >>>

Websites or books having >>> with Python code denote that it is being run on an interactive interpreter session

While defining an indented code block(or continuation lines) the prompt changes to ... (three dots or ellipsis, also called secondary prompt)

Code entered in a secondary prompt has to be indented or else error is thrown

Secondary prompt appears after a statement which requires an indented block, is entered, like conditional

statements (if, for) or function definition (def) statements

>>> the_world_is_round = True
>>> the_world_is_round
True
>>> def indentrospect():
...     if the_world_is_round:
...         print("What is flat?")
...
>>> indentrospect()
What is flat?

As shown, nested blocks require additional indentation

Running code in interactive session

Every expression and statement entered in an interactive session is evaluated and executed immediately

An interactive session allow to test built-in methods, keywords or any piece of code of a Python module or file

It can be handy in running scripts or experimenting with the language

It allows to import modules

>>>import math
>>>math.sqrt(81)
9.0

Exiting from interactive session

To quit the interactive mode use either of

• Ctrl + d shortcut
• Type quit() and enter

Run or execute a Python File

To run a python file, the name of the file is entered along with python command

The file should have ".py" extension

• Open terminal or command-line and use cd to go to the directory containing the python file(if not already in)
• Type python followed by filename and press Enter

That should execute the file (assuming there is no error in its code)

Consider a file named nc_time.py contains following code:

from datetime import datetime
while True:
    t = datetime.time(datetime.now())
    print("Current time is:", t.isoformat())
    input()

Follow above steps to go to directory containing nc_test.py and execute the file with command

python nc_time.py

It should show something like

This program runs in a continuous loop and prints time whenever user presses a key in terminal (except ctrl keys etc)

Press Ctrl + d to quit program