Type Annotations can Cause Unexpected Behavior in Python
,Today I learned that type annotations in Python, while being valid code, can sometimes lead to unexpected problems. In a Django view, I had the following line:
def some_view(request):
...
context["form"]: form
return render(request, some_template, context=context)
This line doesn't raise a SyntaxError because the colon is interpreted as part of a type annotation, which is ignored at runtime. As a result, the line effectively becomes just context["form"], which is a no-op but still valid Python code.
It took me some time to figure out why {% if form.field.errors %} used later on was was always false. 🤦♂️