Lesson 25 - Table Data

Import/Export Design Pattern

This design pattern is used when you want to export a list of objects into a CSV file and import a CSV file to create a list of objects.

The design pattern has several pieces that should be built as stepping stones using test-driven development. Each step is shown by presenting code that demonstrates the pattern.

Table App

Create a new app for table code

$ python manage.py startapp table

config/settings.py

INSTALLED_APPS = [  ..., 'table', ]

CSV Reader

Read a file and convert into a table (list of rows)

Documents/animals.csv

csv 1,Dog 2,Cat 3,Bird 4,Fish

table.py

```python def read_csv_file(path): with open(path) as f: return [row for row in reader(f)]

class TableTest(TestCase):

def test_read_csv(self):
    table = read_csv_file('Documents/animals.csv')
    self.assertEqual(table, [['1', 'Dog'], ['2', 'Cat'], ['3', 'Bird'], ['4', 'Fish']])

```

CSV Writer

Write a table (list of rows) into a CSV file. Each row is a list of values.

```python def export_table(model): records = [o.values() for o in model.objects.all()] write_csv_file('Documents/animals.csv', records)

class TableTest(TestCase):

def test_write_csv(self):
    table = read_csv_file('Documents/animals.csv')
    write_csv_file('Documents/animals.csv', table)
    table = read_csv_file('Documents/animals.csv')
    self.assertEqual(table, [['1', 'Dog'], ['2', 'Cat'], ['3', 'Bird'], ['4', 'Fish']])

```

Chapters in Book Builder

python class Chapter(models.Model): book = models.CharField(max_length=200) order = models.IntegerField() title = models.CharField(max_length=200)

Create object Record

Build a Chapter object from a list of values

```python class Chapter(models.Model):

@staticmethod
def import_record(values):
    c = Chapter.objects.get_or_create(book=values[0], order=values[1])[0]
    c.title = values[2]
    c.save()
    return c

row = ['Tales of Fear', 1, 'Gold Bug'] c = Chapter.import_record(row) assert(c.title == 'Gold Bug') ```

Import from CSV File

For each row in table build one object

python for row in read_csv_file(path): Chapter.import_record(row)

Export to CSV

For each object write one line of CSV data

```python class Chapter(models.Model):

def export_record(self):
    return [self.book, self.order, self.title]

chapters = Chapter.objects.filter(book=book) table = [c.export_record(c) for c in chapters] write_csv_file(path, table) ```

Import/Export Test

Reading objects and them writing them should not change the CSV file

```python class ChapterDataTest(TestCase):

def test_chapter_import_export(self):
    book = Book.objects.get_or_create(title="The Leverage Principle", 
                                      author='Mark Seaman', 
                                      doc_path='Documents/Leverage')[0]
    first = import_chapters(book)
    export_chapters(book)
    second = import_chapters(book)
    self.assertEqual(len(Chapter.objects.all()), 15)
    self.assertEqual(second, first)

```

Executable Django Command

Run Custom Commands

$ python manage.py quick_test

table/management/commands/quick_test.py

```python from django.core.management.base import BaseCommand

from book.book import import_chapters from book.models import Book

class Command(BaseCommand):

def handle(self, *args, **options):
    print("Quick Test")

    for b in Book.objects.all():
        print(import_chapters(b))

```