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chapter 12, module os updated
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natenka committed Jul 14, 2022
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os
---------
===

Module ``os`` allows working with filesystem, environment and managing processes.
The os module allows you to work with the file system, with the environment,
and manage processes.

This subsection addresses only several useful features. For a more complete description of capabilities of module please refer to
`documentation <https://docs.python.org/3/library/os.html>`__ or
`article on Pymotw <https://pymotw.com/3/os/>`__.
This subsection covers only a few useful features. For a more complete
description of the module's capabilities, see the `documentation
<https://docs.python.org/3/library/os.html>`__ or `article on pymotw
<https://pymotw.com/3/os/>`__.

Module ``os`` allows you to create directories:

Module import:

.. code:: python
In [1]: import os
os.environ
----------

os.environ returns a dictionary with environment variables and their values.
You can use square brackets to get variable if the environment
variable definitely exists (an exception will be raised if the variable does
not exist).

.. code:: python
In [2]: os.environ["HOME"]
Out[2]: '/home/nata'
In [3]: os.environ["TOKEN"]
---------------------------------------------------------------------------
KeyError Traceback (most recent call last)
Input In [3], in <cell line: 1>()
----> 1 os.environ["TOKEN"]
File ~/venv/pyneng-py3-8-0/lib/python3.8/os.py:673, in _Environ.__getitem__(self, key)
670 value = self._data[self.encodekey(key)]
671 except KeyError:
672 # raise KeyError with the original key value
--> 673 raise KeyError(key) from None
674 return self.decodevalue(value)
KeyError: 'TOKEN'
Or use the get method, then if there is no environment variable, None is returned:

.. code:: python
In [3]: os.environ.get("HOME")
Out[3]: '/home/nata'
In [4]: os.environ.get("TOKEN")
.. note::

Technically, os.environ returns an object of type `mapping
<https://docs.python.org/3/glossary.html#term-mapping>`__, but at this
point it's easier to think of it as a dictionary.


Environment variables are read the first time the os module is imported.
If some variables were added while the script was running, they will not be
available through ``os.environ``.

os.mkdir
--------

os.mkdir allows you to create a directory:

.. code:: python
In [2]: os.mkdir('test')
In [3]: ls -ls
total 0
0 drwxr-xr-x 2 nata nata 68 Jan 23 18:58 test/
In addition, module contains relevant existence checks. For example, if you try to re-create a directory, an error will occur:
os.listdir
----------

The listdir function returns a list of files and subdirectories in the
specified directory. The order of the files in the list is arbitrary, if you
need to sort them, you can use the sorted function.

.. code:: python
In [4]: os.mkdir('test')
---------------------------------------------------------------------------
FileExistsError Traceback (most recent call last)
<ipython-input-4-cbf3b897c095> in <module>()
----> 1 os.mkdir('test')
In [2]: os.listdir("09_functions")
Out[2]:
['test_task_9_4.py',
'task_9_2a.py',
'task_9_1a.py',
'test_task_9_2.py',
'task_9_3a.py',
'test_task_9_3a.py',
'task_9_3.py',
'test_task_9_3.py',
'config_sw2.txt',
'test_task_9_2a.py',
'config_sw1.txt',
'test_task_9_1a.py',
'test_task_9_1.py',
'task_9_4.py',
'task_9_1.py',
'config_r1.txt',
'task_9_2.py']
In [3]: sorted(os.listdir("09_functions"))
Out[3]:
['config_r1.txt',
'config_sw1.txt',
'config_sw2.txt',
'task_9_1.py',
'task_9_1a.py',
'task_9_2.py',
'task_9_2a.py',
'task_9_3.py',
'task_9_3a.py',
'task_9_4.py',
'test_task_9_1.py',
'test_task_9_1a.py',
'test_task_9_2.py',
'test_task_9_2a.py',
'test_task_9_3.py',
'test_task_9_3a.py',
'test_task_9_4.py']
The current directory can be specified as ``"."`` or call listdir with no arguments:

.. code:: python
In [7]: os.listdir('.')
Out[7]: ['cover3.png', 'dir2', 'dir3', 'README.txt', 'test']
In [7]: os.listdir()
Out[7]: ['cover3.png', 'dir2', 'dir3', 'README.txt', 'test']
os.path
-------

FileExistsError: [Errno 17] File exists: 'test'
Different operating systems (OS) have different path naming conventions, so
there are multiple versions of the os.path module in the standard library. The
os module automatically loads the necessary part to work with the current OS.
For example, when running the same functions of the os module on Windows and
Linux, different values will be considered as the path separator.

In this case, testing with ``os.path.exists`` is useful:
If you need to work on Linux with Windows paths and vice versa, you can use the
``posixpath``, ``ntpath`` modules instead of ``os.path``.


os.path.exists
~~~~~~~~~~~~~~

The ``os.path.exists`` function checks if the specified path exists
and returns True if it exists and False otherwise:

.. code:: python
Expand All @@ -42,14 +165,41 @@ In this case, testing with ``os.path.exists`` is useful:
...: os.mkdir('test')
...:
Method ``listdir`` allows you to view the content of directory:
os.path.isdir, os.path.isfile
~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

The ``os.path.isdir`` function returns True if the path leads to a directory, False otherwise:

.. code:: python
In [7]: os.listdir('.')
Out[7]: ['cover3.png', 'dir2', 'dir3', 'README.txt', 'test']
In [4]: os.path.isdir("09_functions")
Out[4]: True
In [5]: os.path.isdir("/home/nata/repos/pyneng-tasks/exercises/09_functions/")
Out[5]: True
In [6]: os.path.isdir("/home/nata/repos/pyneng-tasks/exercises/09_functions/task_9_1.py")
Out[6]: False
By checking ``os.path.isdir`` and ``os.path.isfile`` you can get a separate list of files and list of directories:
In [7]: os.path.isdir("09_functions/task_9_1.py")
Out[7]: False
The ``os.path.isfile`` function returns True if the path leads to a file and False otherwise:

.. code:: python
In [9]: os.path.isfile("09_functions/task_9_1.py")
Out[9]: True
In [10]: os.path.isfile("09_functions/")
Out[10]: False
Using the checks ``os.path.isdir``, ``os.path.isfile`` and ``os.listdir``, you
can get lists of files and directories (in the example for the current
directory).

List of directories in the current directory:

.. code:: python
Expand All @@ -58,23 +208,60 @@ By checking ``os.path.isdir`` and ``os.path.isfile`` you can get a separate list
In [9]: dirs
Out[9]: ['dir2', 'dir3', 'test']
List of files in the current directory:

.. code:: python
In [10]: files = [f for f in os.listdir('.') if os.path.isfile(f)]
In [11]: files
Out[11]: ['cover3.png', 'README.txt']
Also in module there are separate methods for working with paths:
os.path.split
~~~~~~~~~~~~~

The os.path.split function splits the path into the "main part" and the end of
the path at the last ``/`` and returns a tuple of two elements. This will
automatically use a backslash for Windows.

If there is no slash at the end of the path, the division will be like this

.. code:: python
In [6]: os.path.split("book/25_additional_info/README.md")
Out[6]: ('book/25_additional_info', 'README.md')
In [8]: os.path.split("book/25_additional_info")
Out[8]: ('book', '25_additional_info')
If the path ends with a slash, the second element of the tuple will be an empty string:

.. code:: python
In [12]: file = 'Programming/PyNEng/book/25_additional_info/README.md'
In [7]: os.path.split("book/25_additional_info/")
Out[7]: ('book/25_additional_info', '')
In [9]: os.path.split("book/")
Out[9]: ('book', '')
If the path ends with a slash, the second element of the tuple will be an empty string:

.. code:: python
In [13]: os.path.basename(file)
Out[13]: 'README.md'
In [39]: os.path.split("README.md")
Out[39]: ('', 'README.md')
os.path.abspath
~~~~~~~~~~~~~~~~

The os.path.abspath function returns the absolute path for the specified file or directory:

.. code:: python
In [14]: os.path.dirname(file)
Out[14]: 'Programming/PyNEng/book/25_additional_info'
In [40]: os.path.abspath("09_functions")
Out[40]: '/home/nata/repos/pyneng-tasks/exercises/09_functions'
In [15]: os.path.split(file)
Out[15]: ('Programming/PyNEng/book/25_additional_info', 'README.md')
In [41]: os.path.abspath(".")
Out[41]: '/home/nata/repos/pyneng-tasks/exercises'

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