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from decimal import Decimal
from flask import Flask, request, jsonify
from qval import validate, qval_curry, Validator, QvalValidationError
from qval.framework_integration import setup_flask_error_handlers
app = Flask(__name__)
# Flask uses one global request object.
# You can automatically provide it to the `@qval()` decorator on each call
# by executing `qval_curry(request)`.
qval = qval_curry(request)
# Setup exception handlers
def division_view():
GET /api/divide?
param a : int
param b : int, nonzero
Example: GET /api/divide?a=10&b=2&token=abcdefghijkl -> 200, {"answer": 5}
# Parameter validation occurs in the context manager.
# If validation fails or user code throws an error, context manager
# will raise InvalidQueryParamException or APIException respectively.
# In Django, these exception will be processed and result
# in error codes (400 and 500) on the client side.
params = (
validate(request, a=int, b=int)
# `b` must be anything but zero
with params as p:
return jsonify({"answer": p.a // p.b})
@qval({"a": float, "b": float})
def exponentiation_view(request, params):
Note that request parameter is now being provided to the view.
GET /api/pow?
param a : float
param b : float
Example: GET /api/pow?a=2&b=3 -> 200, {"answer": 8.0}
Example: GET /api/pow?a=2&b=3000000000000 -> 500,
"error": "An error occurred while processing you request. Please contact the website administrator."
# Here we don't catch the OverflowError if `b` is too big.
# This will result in 500 error on the client side.
return jsonify({"answer": params.a ** params.b})
def price_validator(price: int) -> bool:
A predicate to validate `price` query parameter.
Provides custom error message.
if price <= 0:
# If price does not match our requirements, we raise QvalValidationError() with a custom message.
# This exception will be handled in the context manager and will be reraised
# as InvalidQueryParamException() [HTTP 400].
raise QvalValidationError(f"Price must be greater than zero, got '{price}'.")
return True
purchase_factories = {"price": Decimal, "item_id": int, "token": None}
purchase_validators = {
"token": Validator(lambda x: len(x) == 12),
# Validator(p) can be omitted if there is only one predicate:
"item_id": lambda x: x >= 0,
"price": price_validator,
@qval(purchase_factories, purchase_validators)
def purchase_view(request, params):
GET /api/purchase?
param item_id : int, positive
param price : float, greater than zero
param token : string, length == 12
Example: GET /api/purchase?item_id=1&price=5.8&token=abcdefghijkl
-> {"success": "Item '1' has been purchased. Check: 5.92$."
tax = 0.02
cost = params.price * Decimal(1 + tax)
return jsonify(
"success": f"Item '{params.item_id}' has been purchased. Check: {round(cost, 2)}$."
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