145 lines
4.2 KiB
Markdown
145 lines
4.2 KiB
Markdown
[Contents](../Contents) \| [Previous (9.1 Packages)](01_Packages) \| [Next (9.3 Distribution)](03_Distribution)
|
|
|
|
# 9.2 Third Party Modules
|
|
|
|
Python has a large library of built-in modules (*batteries included*).
|
|
|
|
There are even more third party modules. Check them in the [Python Package Index](https://pypi.org/) or PyPi.
|
|
Or just do a Google search for a specific topic.
|
|
|
|
How to handle third-party dependencies is an ever-evolving topic with
|
|
Python. This section merely covers the basics to help you wrap
|
|
your brain around how it works.
|
|
|
|
### The Module Search Path
|
|
|
|
`sys.path` is a directory that contains the list of all directories
|
|
checked by the `import` statement. Look at it:
|
|
|
|
```python
|
|
>>> import sys
|
|
>>> sys.path
|
|
... look at the result ...
|
|
>>>
|
|
```
|
|
|
|
If you import something and it's not located in one of those
|
|
directories, you will get an `ImportError` exception.
|
|
|
|
### Standard Library Modules
|
|
|
|
Modules from Python's standard library usually come from a location
|
|
such as `/usr/local/lib/python3.6'. You can find out for certain
|
|
by trying a short test:
|
|
|
|
```python
|
|
>>> import re
|
|
>>> re
|
|
<module 're' from '/usr/local/lib/python3.6/re.py'>
|
|
>>>
|
|
```
|
|
|
|
Simply looking at a module in the REPL is a good debugging tip
|
|
to know about. It will show you the location of the file.
|
|
|
|
### Third-party Modules
|
|
|
|
Third party modules are usually located in a dedicated
|
|
`site-packages` directory. You'll see it if you perform
|
|
the same steps as above:
|
|
|
|
```python
|
|
>>> import numpy
|
|
<module 'numpy' from '/usr/local/lib/python3.6/site-packages/numpy/__init__.py'>
|
|
>>>
|
|
```
|
|
|
|
Again, looking at a module is a good debugging tip if you're
|
|
trying to figure out why something related to `import` isn't working
|
|
as expected.
|
|
|
|
### Installing Modules
|
|
|
|
The most common technique for installing a third-party module is to use
|
|
`pip`. For example:
|
|
|
|
```bash
|
|
bash % python3 -m pip install packagename
|
|
```
|
|
|
|
This command will download the package and install it in the `site-packages`
|
|
directory.
|
|
|
|
### Problems
|
|
|
|
* You may be using an installation of Python that you don't directly control.
|
|
* A corporate approved installation
|
|
* You're using the Python version that comes with the OS.
|
|
* You might not have permission to install global packages in the computer.
|
|
* There might be other dependencies.
|
|
|
|
### Virtual Environments
|
|
|
|
A common solution to package installation issues is to create a
|
|
so-called "virtual environment" for yourself. Naturally, there is no
|
|
"one way" to do this--in fact, there are several competing tools and
|
|
techniques. However, if you are using a standard Python installation,
|
|
you can try typing this:
|
|
|
|
```bash
|
|
bash % python -m venv mypython
|
|
bash %
|
|
```
|
|
|
|
After a few moments of waiting, you will have a new directory
|
|
`mypython` that's your own little Python install. Within that
|
|
directory you'll find a `bin/` directory (Unix) or a `Scripts/`
|
|
directory (Windows). If you run the `activate` script found there, it
|
|
will "activate" this version of Python, making it the default `python`
|
|
command for the shell. For example:
|
|
|
|
```bash
|
|
bash % source mypython/bin/activate
|
|
(mypython) bash %
|
|
```
|
|
|
|
From here, you can now start installing Python packages for yourself.
|
|
For example:
|
|
|
|
```
|
|
(mypython) bash % python -m pip install pandas
|
|
...
|
|
```
|
|
|
|
For the purposes of experimenting and trying out different
|
|
packages, a virtual environment will usually work fine. If,
|
|
on the other hand, you're creating an application and it
|
|
has specific package dependencies, that is a slightly
|
|
different problem.
|
|
|
|
### Handling Third-Party Dependencies in Your Application
|
|
|
|
If you have written an application and it has specific third-party
|
|
dependencies, one challange concerns the creation and preservation of
|
|
the environment that includes your code and the dependencies. Sadly,
|
|
this has been an area of great confusion and frequent change over
|
|
Python's lifetime. It continues to evolve even now.
|
|
|
|
Rather than provide information that's bound to be out of date soon,
|
|
I refer you to the [Python Packaging User Guide](https://packaging.python.org).
|
|
|
|
## Exercises
|
|
|
|
### Exercise 9.4 : Creating a Virtual Environment
|
|
|
|
See if you can recreate the steps of making a virtual environment and installing
|
|
pandas into it as shown above.
|
|
|
|
[Contents](../Contents) \| [Previous (9.1 Packages)](01_Packages) \| [Next (9.3 Distribution)](03_Distribution)
|
|
|
|
|
|
|
|
|
|
|
|
|