create a class only if a package is available on the system

Question

I have classes in a Python project that depend on an external packages. I would like these classes to be created only if their dependencies are available.

For example, how can I have a class YamlParser which only exists if yaml can be imported?

Answer 1

You can do this within a try - except block.

However, this can highly complicate things if you want to access the class in other places, as the error handling will become more and more complex.

try:
    import yaml
    Class YamlParser():
        pass
except ImportError:
    pass
    #error handling here

You can also see How to check if a python module exists without importing it for ways to do this without the import statement.

Answer 2

Use try and except to handle this cases:

try:
    import yaml
    ## your code goes here
except ImportError:
    ## here you handle the expcetion

Answer 3

The modular and extensible solution is to put YamlParser in its own source file, and simply put the import yaml statement at the beginning. Any code which tries to import this code will fail if the required module yaml is missing.