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Exercises/ex9_4.md

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+\[ [Index](index.md) | [Exercise 9.3](ex9_3.md) | []() \]
+
+# 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.
+
+\[ [Solution](soln9_4.md) | [Index](index.md) | [Exercise 9.3](ex9_3.md) \]
+
+----
+`>>>` Advanced Python Mastery  
+`...` A course by [dabeaz](https://www.dabeaz.com)  
+`...` Copyright 2007-2023  
+
+![](https://i.creativecommons.org/l/by-sa/4.0/88x31.png). This work is licensed under a [Creative Commons Attribution-ShareAlike 4.0 International License](http://creativecommons.org/licenses/by-sa/4.0/)