Python Installation and Getting Started

 Installation

• For Ubuntu: Both Python 2.7 and Python 3.4 should have already installed by default. If not, you can install via:

  # Installing Python 2
  $ sudo apt-get install python
  # Installing Python 3
  $ sudo apt-get install python3
  

  To verify the Python installation (under Ubuntu 14.04 LTS):

  # List packages beginning with python
  $ dpkg --list python*
  ||/ Name                Version        Architecture  
  +++-===================-==============-==============
  ii  python              2.7.5-5ubuntu3 amd64
  ii  python2.7           2.7.6-8ubuntu0 amd64
  ii  python3             3.4.0-0ubuntu2 amd64
  rc  python3.3           3.3.2-7ubuntu3 amd64
  ii  python3.4           3.4.3-1ubuntu1 amd64
  # Locate the Python Interpreters from the PATH
  $ which python*
  /usr/bin/python
  /usr/bin/python2.7
  /usr/bin/python3
  /usr/bin/python3.4
  $ ll /usr/bin/python*
  lrwxrwxrwx 1 root root       9 Dec 21  2013 python -> python2.7*
  lrwxrwxrwx 1 root root       9 Dec 21  2013 python2 -> python2.7*
  -rwxr-xr-x 1 root root 3345416 Jun 23 02:51 python2.7*
  lrwxrwxrwx 1 root root       9 Mar 23  2014 python3 -> python3.4*
  -rwxr-xr-x 2 root root 3709944 Oct 15 05:42 python3.4*
  -rwxr-xr-x 2 root root 3709944 Oct 15 05:42 python3.4m*
  lrwxrwxrwx 1 root root      10 Mar 23  2014 python3m -> python3.4m*
        # Clearly,
        # "python" and "python2" are symlinks to python2.7.
        # "python3" is a simlink to python3.4.
  # Show status of a specific package
  $ dpkg --status python2.7
  Version: 2.7.6-8ubuntu0.2
  ......
  $ dpkg --status python3.4
  Version: 3.4.3-1ubuntu1~14.04.3
  ......

• For Windows: From http://www.python.org/download/, download the 32-bit or 64-bit MSI installer, and run the downloaded installer.

$ python --version
Python 2.7.6
$ python2 -V
Python 2.7.6
$ python3 --version
Python 3.4.3

Getting Started

Interactive Python Command-Line Shell

• In Ubuntu:

  $ python
  Python 2.7.6 (default, Jun 22 2015, 17:58:13) 
  [GCC 4.8.2] on linux2
  Type "help", "copyright", "credits" or "license" for more information.
  >>>

• In Windows: Click the START button ⇒ Python ⇒ Python (Command-line); or run "python.exe" from the Python installed directory.
• In Mac: [TODO]

First Python Script - hello.py

1
2
3
4
5

print('Hello, world')         # Print a string
print(2 ** 88)                # Print 2 raises to the power of 88
                              # Python's integer is unlimited in size!
print(8.01234567890123456789) # Print a float
print((1+2j) * (3*4j))        # Python supports complex numbers!

• Statements beginning with a # until the end-of-line are comments.
• The print() function can be used to print a value to the console.
• Python's strings can be enclosed with single quotes '...' or double quotes "..." (Line 1).
• Python's integer is unlimited in size (Line 2).
• Python support floats (Line 4).
• Python supports complex numbers (Line 5) and other high-level data types.
• By convention, Python script (module) filenames are in all-lowercase.

Running Python Scripts

Running Python Scripts via System Command Shell

• In Linux/Mac Terminal/Bash Shell:

  $ cd <dirname>     # Change directory to where you stored the script
  $ python hello.py  # Run the script via the Python interpreter

  See below for writing executable Python script in Unix.
• In Windows Command Prompt: Start a CMD by entering "cmd" in the start menu.

  > cd <dirname>     # Change directory to where you stored the script
   
  > python hello.py  # Run the script via the Python Interpreter
  > hello.py         # if ".py" file is associated with Python Interpreter

Unix Executable Shell Script

1. Start with a line beginning with #! (called "hash-bang" or "she-bang"), followed by the full-path name to the Python Interpreter, e.g.,

   #!/usr/bin/python
   print('Hello, world')
   print(2 ** 88)
   print(8.01234567890123456789)
   print((1+2j) * (3*4j))

   To locate the Python Interpreter, use command "which python" or "which python3".
2. Make the file executable via chmod (change file mode) command:

   $ cd /path/to/project-directory
   $ chmod u+x hello.py  # enable executable for user-owner
   $ ls -l hello.py      # list to check the executable flag
   -rwxrw-r-- 1 uuuu gggg 314 Nov  4 13:21 hello.py

3. You can then run the Python script just like any executable program. The system will look for the Python Interpreter from the she-bang line.

   $ cd /path/to/project-directory
   $ ./hello.py

Running Python Scripts inside Python Command-Line Shell

• You can use either absolute or relative path for the filename. But, '~' (for home directory) does not work?!
• The open() function open the file, in default read-only mode.
• The read() function reads the entire file.

Environment Variables PATH and PYTHONPATH, and sys.path