Hello world!

The beginning of almost everything in computer programming :-)

Let's see different alternatives to run Python code.

"Batch" running of single commands.
Interacting with Jupyter Notebooks.
Interacting with Python interpreters.
"Batch" running of scripts (modules).

1. "Batch" execution of single Python commands



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!python -c "print('Hello world!')"

2. Interacting with Jupyter Notebook

This interface (what you are reading now) is know as Jupyter Notebook, an interactive document, which is a mix of Markdown and Python code executed by IPython:



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print("Hello world!") # Modify me and push <SHIFT> + <RETURN>

3. Interacting with the Python interpreter

Run python in a shell and type: print("Hello world!") <enter> quit().

$ python
Python 2.7.10 (default, Oct 23 2015, 19:19:21) 
[GCC 4.2.1 Compatible Apple LLVM 7.0.0 (clang-700.0.59.5)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> print("Hello world!")
Hello world!
>>> quit()
$

Alternatively, instead of python we can use ipython, which provides dynamic object introspection, command completion, access to the system shell, etc.

$ ipython
Python 3.5.1rc1 (v3.5.1rc1:948ef16a6951, Nov 22 2015, 11:29:13) 
Type "copyright", "credits" or "license" for more information.

IPython 5.1.0 -- An enhanced Interactive Python.
?         -> Introduction and overview of IPython's features.
%quickref -> Quick reference.
help      -> Python's own help system.
object?   -> Details about 'object', use 'object??' for extra details.

In [1]: print("Hello world!")
Hello world!

In [2]: help(print)
Help on built-in function print in module builtins:

print(...)
    print(value, ..., sep=' ', end='\n', file=sys.stdout, flush=False)

    Prints the values to a stream, or to sys.stdout by default.
    Optional keyword arguments:
    file:  a file-like object (stream); defaults to the current sys.stdout.
    sep:   string inserted between values, default a space.
    end:   string appended after the last value, default a newline.
    flush: whether to forcibly flush the stream.

(type: <q> to exit)

In [3]: quit() # <ctrl> + <d> also works in Unixes
$

Interpreted?

Python is an interpreted programming language. When we run a Python program, we are executing the translation to bytecode of each Python statement of our program over the Python Virtual Machine (PVM). The .pyc files that appear after running a collection of modules as a script for the first time, contains the bytecode of such modules. This is used by Python to speed up their future executions.



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def hello():
    print('Hello world!')
import dis

dis.dis(hello)

4. Running Python programs (modules) as scripts



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!cat hello_world.py



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# Check the code (optional)
!pyflakes3 hello_world.py



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!./hello_world.py # Specific of Unix



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!python hello_world.py