def do_something(params):
    val_a_must_int = params.get('a', 0)
    val_b_must_be_non_empty_list = params.get('b', [])
    # if key c presents, value c must be a date string between '2000-01-01' to '2020-01-01'
    val_c_might_be_none = params.get('c', None)

    # check type
    if type(val_a_must_int) != int:
      raise XXX

    # check type & value
    if type(val_b_must_list) != list or len(val_b_must_be_non_empty_list) == 0:
      raise XXX

    # if value exists, check its value
    if val_c_might_be_none is not None:
        date_c = datetime.strptime(val_c_might_be_present, '%Y-%m-%d')
        date_20000101 = datetime.date(2000, 1, 1)
        date_20200101 = datetime.date(2020, 1, 1)
        if not (date_20000101 <= date_c <= date_20200101):
          raise XXX
    ...
    # do something actually

from data_spec_validator.spec import INT, DIGIT_STR, ONE_OF, Checker, CheckerOP, validate_data_spec

class SomeSpec:
    field_a = Checker([INT])
    field_b = Checker([DIGIT_STR], optional=True)
    field_c = Checker([DIGIT_STR, INT], op=CheckerOP.ANY)

some_data = dict(field_a=4, field_b='3', field_c=1, field_dont_care=[5,6])
validate_data_spec(some_data, SomeSpec) # return True

some_data = dict(field_a=4, field_c='1')
validate_data_spec(some_data, SomeSpec) # return True

some_data = dict(field_a=4, field_c=1)
validate_data_spec(some_data, SomeSpec) # return True

some_data = dict(field_a='4', field_c='1')
validate_data_spec(some_data, SomeSpec) # raise Exception

some_data = dict(field_a='4', field_c='1')
validate_data_spec(some_data, SomeSpec, nothrow=True) # return False

class AnotherSpec:
    field = Checker([ONE_OF], extra={ONE_OF: [1, '2', [3, 4], {'5': 6}]})

another_data = dict(field=[3, 4])
validate_data_spec(another_data, AnotherSpec) # return True

another_data = dict(field='4')
validate_data_spec(another_data, AnotherSpec) # raise Exception

from rest_framework.decorators import api_view
from rest_framework.views import APIView

from data_spec_validator.decorator import dsv
from data_spec_validator.spec import UUID, EMAIL, Checker

class SomeViewSpec:
  param_a = Checker([UUID])
  param_b = Checker([EMAIL])

class SomeView(APIView):
    @dsv(SomeViewSpec)
    def get(self, request):
        pass

@api_view(('POST',))
@dsv(SomeViewSpec)
def customer_create(request):
    pass

gt_check = 'gt_check'
from data_spec_validator.spec.defines import BaseValidator
class GreaterThanValidator(BaseValidator):
    name = gt_check

    @staticmethod
    def validate(value, extra, data):
        criteria = extra.get(GreaterThanValidator.name)
        return value > criteria, ValueError(f'{value} is not greater than {criteria}')

from data_spec_validator.spec import custom_spec, Checker, validate_data_spec
custom_spec.register(dict(gt_check=GreaterThanValidator()))

class GreaterThanSpec:
    key = Checker([gt_check], extra={gt_check: 10})

ok_data = dict(key=11)
validate_data_spec(ok_data, GreaterThanSpec) # return True

nok_data = dict(key=9)
validate_data_spec(ok_data, GreaterThanSpec) # raise Exception

# In default mode, any exception happens, there will be a reason in the message
"field: XXX, reason: '3' is not a integer"

# In vague mode, any exception happens, a general message is shown
"field: XXX not well-formatted"