Hello World¶

Hello World: Interactive Mode
Hello World: Python 3
Hello World: Python 2 vs. Python 3
Hello World: Script Files

Hello World: Interactive Mode ¶

Interactive Mode

Python interpreter, invoked without arguments
“Shell prompt”, only with Python
Exit ⟶ type Ctrl + d (End-of-file)

$ python
Python 2.7.9 (default, Aug 15 2015, 22:03:50)
[GCC 4.8.4] on linux2
Type "help", "copyright", "credits" or "license" for ...
>>> print "Hello World"
Hello World
>>>

Hello World: Python 3 ¶

Major annoyance: Python 3 is not compatible with Python 2

Breaking compatibility is not an easy decision
Necessary (so they say) to clean up > 20 years of dirt
First hurdle: print is a statement in 2, and a function in 3

$ python3
Python 3.4.1 (default, Aug 15 2015, 22:12:12)
[GCC 4.8.4] on linux
Type "help", "copyright", "credits" or "license" for ...
>>> print("Hello World")
Hello World
>>>

Hello World: Python 2 vs. Python 3 ¶

Compatibility strategy: the __future__ module

Enable future features in current versions
Clearly remains valid in the future version
One of many strategies
The remainder of the course will try to be …
- version agnostic
- forward compatible
- backward compatible

Tataa: the feature print_function!

$ python2
...
>>> from __future__ import print_function
>>> print("Hello World")
Hello World
>>>

Hello World: Script Files ¶

The first and simplest program …

hello-world.py¶

#!/usr/bin/python

# omitted from now on
from __future__ import print_function

print("Hello World")

Make it executable, execute …

$ chmod +x hello-world.py
$ ./hello-world.py