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Exercises/soln9_3.md

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+# Exercise 9.3 - Solution
+
+## (a) Controlling Exported Symbols
+
+```python
+# structure.py
+
+__all__ = ['Structure']
+...
+```
+
+```python
+# reader.py
+
+__all__ = [ 'read_csv_as_dicts',
+            'read_csv_as_instances' ]
+...
+```
+
+```python
+# tableformat.py
+
+__all__ = ['create_formatter', 'print_table']
+...
+```
+
+## (b) Exporting Everything
+
+```python
+# structly/__init__.py
+
+from .structure import *
+from .reader import *
+from .tableformat import *
+
+__all__ = [ *structure.__all__,
+	    *reader.__all__,
+	    *tableformat.__all__ ]
+```
+
+## (c) Module Splitting
+
+There are a few parts to this.   First, the individual files in `tableformat/formats` are going to
+look like this:
+
+```python
+# tableformat/formats/text.py
+from ..formatter import TableFormatter
+
+class TextTableFormatter(TableFormatter):
+    ...
+
+
+# tableformat/formats/csv.py
+from ..formatter import TableFormatter
+class CSVTableFormatter(TableFormatter):
+    ...
+
+
+# tableformat/formats/html.py
+from ..formatter import TableFormatter
+class HTMLTableFormatter(TableFormatter):
+    ...
+```
+
+The `formatter.py` file will look like this:
+
+```python
+# tableformat/formatter.py
+
+from abc import ABC, abstractmethod
+
+def print_table(records, fields, formatter):
+    if not isinstance(formatter, TableFormatter):
+        raise RuntimeError('Expected a TableFormatter')
+
+    formatter.headings(fields)
+    for r in records:
+        rowdata = [getattr(r, fieldname) for fieldname in fields]
+        formatter.row(rowdata)
+
+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
+
+class ColumnFormatMixin:
+    formats = []
+    def row(self, rowdata):
+        rowdata = [ (fmt % item) for fmt, item in zip(self.formats, rowdata)]
+        super().row(rowdata)
+
+class UpperHeadersMixin:
+    def headings(self, headers):
+        super().headings([h.upper() for h in headers])
+
+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()
+```
+
+Finally, you need to have a `tableformat/__init__.py` file like this:
+
+```python
+# tableformat/__init__.py
+
+from .formatter import print_table, create_formatter
+
+__all__ = [ 'print_table', 'create_formatter' ]
+```
+
+
+[Back](ex9_3.md)