Recap & Outlook

So far, we have learned how to use Python for programming, focused on Python’s basic data types, modules, built-in functions, manipulations of them and also the mathematical programming and plotting using numpy and matplotlib.

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.

Reading Materials

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

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)
<class '_io.TextIOWrapper'>
>>> f.write('Welcome to AMS 129 \nwe love scientific computing!')
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>>> f.close()

To read from a file

>>> f=open('output','r')
>>> fread=f.read()
>>> print(fread)
Welcome to AMS 129
we love scientific computing!
>>> f.close()

To add more lines, open the file with mode a as a second parameter for append

>>> f=open('output','w')
>>> f.write('Welcome to AMS 129 \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 129
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

>>> f=open('output','r')
>>> for line in f:
...     print(line)
...
Welcome to AMS 129

we love scientific computing!

Python is fun to learn

>>> f.close()

You may notice that a blank line has been inserted in every line variable. This is because the escape character, \n, is recognized as a blank line. If you want to delete the escape character, you may use the strip method.

>>> f = open('output', 'r')
>>> for line in f:
...     print(line.strip())
...
Welcome to AMS 129
we love scientific computing!
Python is fun to learn
>>> 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)

with statement

In the previous examples, you may noticed that the general code structure of the input/output of the Python. For example,

f=open(file_name, file_mode)
# Do something with file as f
f.close()

Actually, you don’t need to close the file. When your python script is over, the Python interpreter will close the file automatically, even though there is no close method given. However, by writing the close statement, your code is more easy to follow, (to human) and also it prevents the further error(s).

Practically, a lot of Python programmer loves to handle the file with with statement.

with open(file_name, file_mode) as f:
   # Do something with file as f
   # If exit this block, the file will be closed automatically.

In this manner, all you file handling code will be presented inside of this block, and you can assume f.close() statement by exiting the block.

Command line input arguments

If you want to give input argument(s) to your program with command line, say, for instance,

$ ./your_python_program.py YourArgument1 YourArgument2

In this case, you can use the sys module.

import sys

arg1 = sys.argv[1]
arg2 = sys.argv[2]
...

As you can see in the above example, your first command line input argument is taken as sys.argv[1]. But wait, the index for Python start from 0. Then, what is the sys.argv[0]? The answer is simple; the file name of your program itself. See the following example.

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import sys

# All input arguments will be stored in
# sys.argv list.
for arg in sys.argv:
    print(arg)

If you execute this file as,

$ python3 sys_arg.py aa bb cc
sys_arg.py
aa
bb
cc

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.