'''Internal lists of allowed and denied methods.

These are lists of objects and each object has 2 properties: A resource
ARN and a nullable conditions statement. The build method processes these
lists and generates the approriate statements for the final policy.
'''
allowMethods = []
denyMethods = []

# The API Gateway API id. By default this is set to '*'
restApiId = '*'
# The region where the API is deployed. By default this is set to '*'
region = '*'
# The name of the stage used in the policy. By default this is set to '*'
stage = '*'

def __init__(self, principal, awsAccountId):
self.awsAccountId = awsAccountId
self.principalId = principal
self.allowMethods = []
self.denyMethods = []

def _addMethod(self, effect, verb, resource, conditions):
'''Adds a method to the internal lists of allowed or denied methods. Each object in
the internal list contains a resource ARN and a condition statement. The condition
statement can be null.'''
if verb != '*' and not hasattr(HttpVerb, verb):
raise NameError('Invalid HTTP verb ' + verb + '. Allowed verbs in HttpVerb class')
resourcePattern = re.compile(self.pathRegex)
if not resourcePattern.match(resource):
raise NameError('Invalid resource path: ' + resource + '. Path should match ' + self.pathRegex)

if resource[:1] == '/':
resource = resource[1:]

resourceArn = 'arn:aws:execute-api:{}:{}:{}/{}/{}/{}'.format(self.region, self.awsAccountId, self.restApiId, self.stage, verb, resource)

if effect.lower() == 'allow':
self.allowMethods.append({
'resourceArn': resourceArn,
'conditions': conditions
})
elif effect.lower() == 'deny':
self.denyMethods.append({
'resourceArn': resourceArn,
'conditions': conditions
})

def _getEmptyStatement(self, effect):
'''Returns an empty statement object prepopulated with the correct action and the
desired effect.'''
statement = {
'Action': 'execute-api:Invoke',
'Effect': effect[:1].upper() + effect[1:].lower(),
'Resource': []
}

return statement

def _getStatementForEffect(self, effect, methods):
'''This function loops over an array of objects containing a resourceArn and
conditions statement and generates the array of statements for the policy.'''
statements = []

if len(methods) > 0:
statement = self._getEmptyStatement(effect)

for curMethod in methods:
if curMethod['conditions'] is None or len(curMethod['conditions']) == 0:
statement['Resource'].append(curMethod['resourceArn'])
else:
conditionalStatement = self._getEmptyStatement(effect)
conditionalStatement['Resource'].append(curMethod['resourceArn'])
conditionalStatement['Condition'] = curMethod['conditions']
statements.append(conditionalStatement)

if statement['Resource']:
statements.append(statement)

return statements

def allowAllMethods(self):
'''Adds a '*' allow to the policy to authorize access to all methods of an API'''
self._addMethod('Allow', HttpVerb.ALL, '*', [])

def denyAllMethods(self):
'''Adds a '*' allow to the policy to deny access to all methods of an API'''
self._addMethod('Deny', HttpVerb.ALL, '*', [])

def allowMethod(self, verb, resource):
'''Adds an API Gateway method (Http verb + Resource path) to the list of allowed
methods for the policy'''
self._addMethod('Allow', verb, resource, [])

def denyMethod(self, verb, resource):
'''Adds an API Gateway method (Http verb + Resource path) to the list of denied
methods for the policy'''
self._addMethod('Deny', verb, resource, [])

def allowMethodWithConditions(self, verb, resource, conditions):
'''Adds an API Gateway method (Http verb + Resource path) to the list of allowed
methods and includes a condition for the policy statement. More on AWS policy
conditions here: http://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_elements.html#Condition'''
self._addMethod('Allow', verb, resource, conditions)

def denyMethodWithConditions(self, verb, resource, conditions):
'''Adds an API Gateway method (Http verb + Resource path) to the list of denied
methods and includes a condition for the policy statement. More on AWS policy
conditions here: http://docs.aws.amazon.com/IAM/latest/UserGuide/reference_policies_elements.html#Condition'''
self._addMethod('Deny', verb, resource, conditions)

def build(self):
'''Generates the policy document based on the internal lists of allowed and denied
conditions. This will generate a policy with two main statements for the effect:
one statement for Allow and one statement for Deny.
Methods that includes conditions will have their own statement in the policy.'''
if ((self.allowMethods is None or len(self.allowMethods) == 0) and
(self.denyMethods is None or len(self.denyMethods) == 0)):
raise NameError('No statements defined for the policy')

policy = {
'principalId': self.principalId,
'policyDocument': {
'Version': self.version,
'Statement': []
}
}

policy['policyDocument']['Statement'].extend(self._getStatementForEffect('Allow', self.allowMethods))
policy['policyDocument']['Statement'].extend(self._getStatementForEffect('Deny', self.denyMethods))