## Filesystem Operations
* A file is stored as a resource on the computer on top of a [filesystem](https://www.tldp.org/LDP/sag/html/filesystems.html), which is a storage format.
* Files can be text, music or media which can be understood by any human user.
* And some files are binaries which can only be understood by computer.
* By using Python, we can easily manipulate these files.
* Let's create a file - `names.txt` and add few names in it.
* list of different modes for opening files:
* `r` - read the content of file, setted default
* `w` - write the content to file, if file is not there it will create it.
* `a` - append contents to the file.
```
>>> names_list = ['chandan', 'nikhil']
>>> data = open('names.txt', 'w')
>>> for i in names_list:
...     data.write(i)
... 
7
6
>>> data.close()
>>> data = open('names.txt', 'r')
>>> data.read()
'chandannikhil'
>>> data.close()
>>> data = open('names.txt', 'a')
>>> data.write('\nBhavin gandahi')
15
>>> data.close()
>>> data = open('names.txt')
>>> data.read()
'chandannikhil\nBhavin gandahi'
>>> data.readline()
''
>>> with open('foo.txt', 'w') as f:
...     f.write("This is awesome")
... 
```

## Python Modules
* Python provides a rich support for python modules.
* Modules/libraries (also known as python packages) are a collection of python scripts which can be reusable for writing complex programs.
* We can import a module using `import` keyword.
* If it is not available it will give import error.
* dir() method tells what the attribute provides.
* What's The [Zen](https://www.python.org/dev/peps/pep-0020/) of Python ? 
```
>>> import this
The Zen of Python, by Tim Peters

Beautiful is better than ugly.
Explicit is better than implicit.
Simple is better than complex.
Complex is better than complicated.
Flat is better than nested.
Sparse is better than dense.
Readability counts.
Special cases aren't special enough to break the rules.
Although practicality beats purity.
Errors should never pass silently.
Unless explicitly silenced.
In the face of ambiguity, refuse the temptation to guess.
There should be one-- and preferably only one --obvious way to do it.
Although that way may not be obvious at first unless you're Dutch.
Now is better than never.
Although never is often better than *right* now.
If the implementation is hard to explain, it's a bad idea.
If the implementation is easy to explain, it may be a good idea.
Namespaces are one honking great idea -- let's do more of those!

>>> import math
>>> dir(math)
['__doc__', '__file__', '__loader__', '__name__', '__package__', '__spec__', 'acos', 'acosh', 'asin', 'asinh', 'atan', 'atan2', 'atanh', 'ceil', 'copysign', 'cos', 'cosh', 'degrees', 'e', 'erf', 'erfc', 'exp', 'expm1', 'fabs', 'factorial', 'floor', 'fmod', 'frexp', 'fsum', 'gamma', 'gcd', 'hypot', 'inf', 'isclose', 'isfinite', 'isinf', 'isnan', 'ldexp', 'lgamma', 'log', 'log10', 'log1p', 'log2', 'modf', 'nan', 'pi', 'pow', 'radians', 'remainder', 'sin', 'sinh', 'sqrt', 'tan', 'tanh', 'tau', 'trunc']
>>> math.e
2.718281828459045
>>> import platform
>>> platform.python_version()
'3.7.3'
>>> help('modules')
>>> import foo
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ModuleNotFoundError: No module named 'foo'
>>> import random
>>> random.randint(1, 100)
79
``` 
* A python script foo.py can be called as a module by using foo.py

## Exceptions
While writing programs, errors are prone to occur. But they can be handled with
exceptions.
For example, we were trying to import a module but it was not there.
We can by pass this error by using try and except.
With `try` keyword, we write something, then in `except` clause once an
exception comes, we handle it or do something else. And using `finally` we can move ahead and do something else.
```
>>> import sys
>>> def linux_interaction():
...     assert ('linux' in sys.platform), "Function can only run on Linux systems."
...     print('Doing something.')
... 
>>> try:
...     linux_interaction()
... except AssertionError as error:
...     print(error)
... else:
...     print('Executing the else clause.')
... 
Doing something.
Executing the else clause.
>>> 
```

## Writting a simple CIL script
* We write scripts in order to reuse it multiple times.
* In python you see '__' double underscore aka dunder.
* Create a script greetings.py and fun.py
```
[raukadah@ironman python-workshop]$ cat greetings.py 
#!/usr/bin/python3
import sys

def greeting(name):
    print("Welcome {0}".format(name))
    print("the value of __name__ is {0}".format( __name__))

if __name__ == "__main__":
    name = "Chandan Kumar"
    greeting(name)

[raukadah@ironman python-workshop]$ python3 greeting.py 
Welcome Chandan Kumar
the value of __name__ is __main__
```
```
[raukadah@ironman python-workshop]$ cat fun.py 
#!/usr/bin/python3

import greetings

greetings.greeting('FOSS MEET')

[raukadah@ironman python-workshop]$ python3 fun.py 
Welcome FOSS MEET
the value of __name__ is greetings
```
* Observe meaning of `__name__` and `__main__`.