Where do I need to place my .py files?

Question

In "Learning python the hard way" the author states we can import a script without having to use python ex25.py. We can do import ex25 and the script will run.

When I do this, I get:

>>> import ex25
Traceback (most recent call last):
  File "<stdin>", line 1, in <modul
ImportError: No module named ex25

The author states it will look like this

Python 2.7.1 (r271:86832, Jun 16 2011, 16:59:05)
[GCC 4.2.1 (Based on Apple Inc. build 5658) (LLVM build 2335.15.00)] on darwin
Type "help", "copyright", "credits" or "license" for more information.
>>> import ex25
>>> sentence = "All good things come to those who wait."
>>> words = ex25.break_words(sentence)
>>> words
['All', 'good', 'things', 'come', 'to', 'those', 'who', 'wait.']
>>> sorted_words = ex25.sort_words(words)
>>> sorted_words
['All', 'come', 'good', 'things', 'those', 'to', 'wait.', 'who']
>>> ex25.print_first_word(words)
All
>>> ex25.print_last_word(words)
wait.
>>> wrods
Traceback (most recent call last):
File "<stdin>", line 1, in <module>
NameError: name 'wrods' is not defined
>>> words
['good', 'things', 'come', 'to', 'those', 'who']
>>> ex25.print_first_word(sorted_words)
All
>>> ex25.print_last_word(sorted_words)
who
>>> sorted_words

It seems that Python is not able to find ex25. Perhaps I need to place it in the Python 27 directory? However, how then, do I keep a clean directory if I need to place all my .py files there?

When I don't get the first challenge (can't find file), I get:

>>> import ex25
>>> 

Seems like nothing is happening.

Answer 1

In your System Config you will find a Variable call PythonPath (Default C:\Python27). When running the python command from the command line it will check in your current directory (Default C:\Users\[Your Name]) and the directory specified by the PythonPath. If you want to run your programm you need to either:

1. Change to the Folder where your Programm is by using ie. cd Desktop
2. Put in in your Python27 Folder

If you are using IDLE you dont need to use the console, you can just press F5 and the programm will run.

Answer 2

If you want to import a package in python (ex25 for example), you should structure your source code in a specific way like:

 .
 └── ex25               # <- package name = folder name
     ├── __init__.py    # <- can be empty
     └── lib.py         # <- contains your class or code

The if you want to import this package you do:

>> import ex25

If you want to import the specific class "DummyClass" in lib file you do:

>> from ex25.lib import DummyClass
>> class B(DummyClass):
     pass

and then continue with your code.

As for python learning books I would recommend that you pick whatever you find it easy to start with and continue to the end with it. A website or a book.

Hope this helps.

Edit: (after editing the question)

I have no idea about that book, but it looks like the books comes with some already written source code and packages to practice on. Try to get that source code and there you should find a folder with the name ``ex25'' .. probably.

Answer 3

Also, current directory on Unix variants, or per shell session one liner:

export PYTHONPATH=$PYTHONPATH:/some/path

Or add that line to .bash_profile, .bashrc, .profile, or whichever is preferred, to have python check the folder each time a shell/terminal gets created.

Or for windows:

set PYTHONPATH=%PYTHONPATH%;C:\path\to\files

And set with autoexec.bat

Better yet, import programatically with:

sys.path.append

Also see:

• Importing files from different folder in Python
• How to add to the pythonpath in windows 7?

Answer 4

You can create packages in Python by putting an empty file entitled __init__.py in the directory containing other Python files. Once you do that, you can collectively or individually import that directory, or any python scripts in that directory, or any modules or functions in the python scripts in that directory. That is, as long as that directory is either located in your Python/lib, or in the directory where the Python script that is importing it, is saved.

The alternative to that would be to add the path where the your library is located to Python's search path. You can do so as follows:

from sys import path
path.append('The absolute path to your module/library')

Answer 5

Example of library creation and module implementation/import in Python.

Hierarchy

.
├── package_a
│   ├── __init__.py
│   ├── mod_a.py
│   └── mod_b.py
└── package_b
    ├── __init__.py
    ├── mod_a2.py
    └── mod_b2.py

Contents

Individual files contain:

File name package_a/__init__.py:

#!/usr/bin/env python3

File name package_a/mode_a.py:

#!/usr/bin/env python3

MOD_A_CONSTANT = 5

File name package_a/mod_b.py:

#!/usr/bin/env python3

from package_a.mod_a import MOD_A_CONSTANT


class MyModClass(object):
        class_prop = 'This is MOD_A_CONSTANT from mod_b class: {}'.format(MOD_A_CONSTANT)

        def __str__(self):
               return self.class_prop


if __name__ == '__main__':
        test = MyModClass()
        print(test)
        # Displays: This is MOD_A_CONSTANT from mod_b class: 5

File name package_b/__init__.py:

#!/usr/bin/env python3

File name package_b/mod_a.py:

#!/usr/bin/env python3

from package_a import mod_a, mod_b

def string_args():
        resp = (
                'mod_a.MOD_A_CONSTANT: {} | '
                'mod_b.MyModClass.class_prop: {}'.format(
                        mod_a.MOD_A_CONSTANT,
                        mod_b.MyModClass.class_prop
                )
        )

        return resp
        # Displays: mod_a.MOD_A_CONSTANT: 5 | mod_b.MyModClass.class_prop: This is MOD_A_CONSTANT from mod_b class: 5


if __name__ == '__main__':
        print(string_args())

File name package/mod_b2.py:

#!/usr/bin/env python3

from package_b.mod_a2 import string_args


class MyMod2Class(object):
        def __str__(self):
                return string_args()


if __name__ == '__main__':
        test = MyMod2Class()
        print(str(test).replace('|', '\n\n'))
        # Displays: 
        # mod_a.MOD_A_CONSTANT: 5 
        #
        # mod_b.MyModClass.class_prop This is MOD_A_CONSTANT from mod_b class: 5

This should tell you all you need to know about creating, importing, and implementing modules and libraries in Python.