python-mastery/Exercises/ex9_4.md

\[ [Index](index.md) | [Exercise 9.3](ex9_3.md) | []() \]

# Exercise 9.4

*Objectives:*

- Explore circular imports
- Dynamic module imports

In the last exercise, you split the `tableformat.py` file up into submodules.
The last part of the resulting `tableformat/formatter.py` file has turned into a mess of imports.

```python
# tableformat.py
...

class TableFormatter(ABC):
    @abstractmethod
    def headings(self, headers):
        pass

    @abstractmethod
    def row(self, rowdata):
        pass

from .formats.text import TextTableFormatter
from .formats.csv import CSVTableFormatter
from .formats.html import HTMLTableFormatter

...

def create_formatter(name, column_formats=None, upper_headers=False):
    if name == 'text':
        formatter_cls = TextTableFormatter
    elif name == 'csv':
        formatter_cls = CSVTableFormatter
    elif name == 'html':
        formatter_cls = HTMLTableFormatter
    else:
        raise RuntimeError('Unknown format %s' % name)

    if column_formats:
        class formatter_cls(ColumnFormatMixin, formatter_cls):
              formats = column_formats

    if upper_headers:
        class formatter_cls(UpperHeadersMixin, formatter_cls):
            pass

    return formatter_cls()
```

The imports in the middle of the file are required because the `create_formatter()`
function needs them to find the appropriate classes.   Really, the whole thing is a mess.

## (a) Circular Imports

Try moving the following import statements to the top of the `formatter.py` file:

```python
# formatter.py

from .formats.text import TextTableFormatter
from .formats.csv import CSVTableFormatter
from .formats.html import HTMLTableFormatter

class TableFormatter(ABC):
    @abstractmethod
    def headings(self, headers):
        pass

    @abstractmethod
    def row(self, rowdata):
        pass

...
```

Observe that nothing works anymore.  Try running the `stock.py` program and
notice the error about `TableFormatter` not being defined.   The order
of import statements matters and you can't just move the imports anywhere
you want.

Move the import statements back where they were.  Sigh.

## (b) Subclass Registration

Try the following experiment and observe:

```python
>>> from structly.tableformat.formats.text import TextTableFormatter
>>> TextTableFormatter.__module__
'structly.tableformat.formats.text'
>>> TextTableFormatter.__module__.split('.')[-1]
'text'
>>>
```

Modify the `TableFormatter` base class by adding a dictionary and an
`__init_subclass__()` method:

```python
class TableFormatter(ABC):
    _formats = { }

    @classmethod
    def __init_subclass__(cls):
        name = cls.__module__.split('.')[-1]
        TableFormatter._formats[name] = cls

    @abstractmethod
    def headings(self, headers):
        pass

    @abstractmethod
    def row(self, rowdata):
        pass
```

This makes the parent class track all of its subclasses.  Check it out:

```python
>>> from structly.tableformat.formatter import TableFormatter
>>> TableFormatter._formats
{'text': <class 'structly.tableformat.formats.text.TextTableFormatter'>, 
 'csv': <class 'structly.tableformat.formats.csv.CSVTableFormatter'>, 
 'html': <class 'structly.tableformat.formats.html.HTMLTableFormatter'>}
>>>
```

Modify the `create_formatter()` function to look up the class in this dictionary
instead:

```python
def create_formatter(name, column_formats=None, upper_headers=False):
    formatter_cls = TableFormatter._formats.get(name)
    if not formatter_cls:
        raise RuntimeError('Unknown format %s' % name)

    if column_formats:
        class formatter_cls(ColumnFormatMixin, formatter_cls):
              formats = column_formats

    if upper_headers:
        class formatter_cls(UpperHeadersMixin, formatter_cls):
            pass

    return formatter_cls()
```

Run the `stock.py` program.  Make sure it still works after you've made these changes.
Just a note that all of the import statements are still there.  You've mainly
just cleaned up the code a bit and eliminated the hard-wired class names.

## (c) Dynamic Imports

You're now ready for the final frontier.   Delete the following import statements
altogether:

```python
# formatter.py
...

from .formats.text import TextTableFormatter     # DELETE
from .formats.csv import CSVTableFormatter       # DELETE
from .formats.html import HTMLTableFormatter     # DELETE
...
```

Run your `stock.py` code again--it should fail with an error. It knows nothing about the
text formatter.  Fix it by adding this tiny fragment of code to `create_formatter()`:

```python
def create_formatter(name, column_formats=None, upper_headers=False):
    if name not in TableFormatter._formats:
        __import__(f'{__package__}.formats.{name}')
    ...
```

This code attempts a dynamic import of a formatter module if nothing is known about the
name.   The import alone (if it works) will register the class with the `_formats` 
dictionary and everything will just work.   Magic!

Try running the `stock.py` code and make sure it works afterwards.

\[ [Solution](soln9_4.md) | [Index](index.md) | [Exercise 9.3](ex9_3.md) \]

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