.. _ch04-python-IO:

===============================================================
Recap & Outlook
===============================================================

So far, we have learned how to use Python for programming, focused on Python's basic
data types, modules, built-in functions, and manipulations of them.

In this short chapter we extend our knoweldge little bit further so
that we can use Python to

1. produce data inputs and outputs,
2. run Python commands to interact with operating systems,
3. learn how to debug efficiently,
4. briefly look at a quick example of object-orient programming.

In the following chapter, we are going to learn mathematical programming and plotting using
`numpy <http://www.numpy.org>`_ and `matplotlib <http://matplotlib.org>`_ in depth.



===============================================================
Reading Materials
===============================================================

The materials of this chapter in part have been adopted and modified from:

 * `Scipy Lecture Note <http://www.scipy-lectures.org>`_,

 * `Think Python by Allen B. Downey
   <http://www.greenteapress.com/thinkpython/thinkpython.pdf>`_,

 * `Prof. LeVeque's (Univ. of Washington) online note on Python 
   <http://faculty.washington.edu/rjl/classes/am583s2014/notes/python.html>`_.




===============================================================
Python inputs and outputs
===============================================================

-------------------------------------------------------------------------------
Read and write files
-------------------------------------------------------------------------------

To be exhaustive, here are some information about input and output in Python.
We write or read *strings* to/from files (other types must be converted to strings).
To write onto a file ::

  >>> f=open('output','w')
  >>> type(f)
  <type 'file'>
  >>> f.write('Welcome to AMS 209 \nwe love scientific computing!')
  >>> f.close()

  
To read from a file ::
  
  >>> f=open('output','r')
  >>> fread=f.read()
  >>> print fread 
  Welcome to AMS 209 
  we love scientific computing!
  >>> f.close()

To add more lines, open the file with mode ``a`` as a second parameter for append::
  
  >>> f.write('Welcome to AMS 209 \nwe love scientific computing!')
  >>> f.close()
  >>> f=open('output','a')
  >>> f.write('\nPython is fun to learn')
  >>> f.close()

To see how it is changed now::
  
  >> f=open('output','r')
  >>> s=f.read()
  >>> print s
  Welcome to AMS 209 
  we love scientific computing!
  Python is fun to learn
  >>> f.close()

  
See also For more details: https://docs.python.org/tutorial/inputoutput.html

-------------------------------------------------------------------------------
Iterating over a file
-------------------------------------------------------------------------------

A file can be used as an iterable in Python::

  >>> for line in f:
  ...     print line
  ... 
  Welcome to AMS 209 
  
  we love scientific computing!

  >>> f.close()


If we do the same file iteration after ``f.close()`` *without* ``f=open('output','r')``, we will
get an error message::

  >>> for line in f:
  ...     print line
  ... 
  Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  ValueError: I/O operation on closed file

  
-------------------------------------------------------------------------------
File modes
-------------------------------------------------------------------------------

* Read-only: ``r``
* Write-only: ``w``
* Append a file: ``a``
* Read and Write: ``r+``
* Binary mode: ``b`` (use for binary files, especially on Windows)



-------------------------------------------------------------------------------
Exercise
-------------------------------------------------------------------------------
1. Write a simple routine that creates ``n`` many files with names
   ``junk_1.txt``, ..., ``junk_n.txt``, each of which contains its
   file name, e.g., ``This is junk_5.txt``.

2. Write a simple routine that reads the contents of the files you just created.