feat: add Currency class

features/steps/hello.py

@@ -1,19 +1,21 @@
 from behave import given, when, then
+from ddd_sample.domain.currency import USD, Currency
 
 
 @given('a currency item in {denomination}')
 def _(context, denomination: str):
-    context.denomination = denomination
+    assert denomination == 'USD-cents', f"Denomination '{denomination}' is currently unsupported"
+    context.format = USD
 
 
 @given('value of {value:d}')
 def _(context, value: int):
-    context.value = value
+    context.currency = Currency(value=value, format=context.format)
 
 
 @when('the currency is stringified')
 def _(context):
-    context.string = ''
+    context.string = str(context.currency)
 
 
 @then('it matches {pattern}')

src/ddd_sample/domain/currency.py

@@ -0,0 +1,18 @@
+from typing import Callable
+
+
+CurrencyFormatMixin = Callable[[int], str]
+
+
+def USD(value: int) -> str:
+    return "${:,}.{:02d}".format(value // 100, value % 100)
+
+
+class Currency:
+    def __init__(self, value: int, format: CurrencyFormatMixin):
+        super().__init__()
+        self.value = value
+        self.format = format
+
+    def __str__(self) -> str:
+        return self.format(self.value)