Sample code for Chapter 10 - "Design patterns with first class functions"
From the book "Fluent Python" by Luciano Ramalho (O'Reilly, 2015)
http://shop.oreilly.com/product/0636920032519.do
Notes
=====
No issues on file with zero type hints
--------------------------------------
Running Mypy on ``classic_strategy.py`` from the first edition, with no
type hints::
$ mypy classic_strategy.py
Success: no issues found in 1 source file
Type inference at play
----------------------
When the ``Order.due`` method made first assignment to discount as ``discount = 0``,
Mypy complained::
mypy classic_strategy.py
classic_strategy.py:68: error: Incompatible types in assignment (expression has type "float", variable has type "int")
Found 1 error in 1 file (checked 1 source file)
To fix it, I made the first assigment as ``discount = 0``.
I never explicitly declared a type for ``discount``.
Mypy ignores functions with no annotations
------------------------------------------
Mypy did not raise any issues with this test case::
def test_bulk_item_promo_with_discount(customer_fidelity_0):
cart = [LineItem('banana', 30, .5),
LineItem('apple', 10, 1.5)]
order = Order(customer_fidelity_0, 10, BulkItemPromo())
assert order.total() == 30.0
assert order.due() == 28.5
The second argument to ``Order`` is declared as ``Sequence[LineItem]``.
Mypy only checks the body of a function the signature as at least one annotation,
like this::
def test_bulk_item_promo_with_discount(customer_fidelity_0) -> None:
cart = [LineItem('banana', 30, .5),
LineItem('apple', 10, 1.5)]
order = Order(customer_fidelity_0, 10, BulkItemPromo())
assert order.total() == 30.0
assert order.due() == 28.5
Now Mypy complains that "Argument 2 of Order has incompatible type".
However, even with the annotation in the test function signature,
Mypy did not find any problem when I mistyped the name of the ``cart`` argument.
Here, ``cart_plain`` should be ``cart``::
def test_bulk_item_promo_with_discount(customer_fidelity_0) -> None:
cart = [LineItem('banana', 30, .5),
LineItem('apple', 10, 1.5)]
order = Order(customer_fidelity_0, cart_plain, BulkItemPromo())
assert order.total() == 30.0
assert order.due() == 28.5
Hypotesis: ``cart_plain`` is a function decorated with ``@pytest.fixture``,
and at the top of the test file I told Mypy to ignore the Pytest import::
import pytest # type: ignore
