Custom exceptions – catching classes that do not inherit from BaseException
Question:
I’m trying to write some custom exception handling but keep running into issues with ‘TypeError: catching classes that do not inherit from BaseException is not allowed’ errors. I have a base exception class called NodeError that inherits from Exception. From there, I have several custom exceptions that inherit from NodeError.
The web3 module uses the requests module to communicate with a node. My test constantly tries to get the tx count from the node and while it does that, I try to simulate an outage by disabling my NIC. I try to catch requests.exceptions.ConnectionError in get_tx_count() and raise my own exception. It seems to correctly hit the NodeConnectionError custom exception based on the stack trace but then gets another exception and complains about catching classes that don’t inherit from BaseException.
Not sure why it thinks I’m not inheriting from BaseException but I have a feeling it has to do with catching the requests exception first.
Stack trace:
Traceback (most recent call last):
File "C:Python39libsite-packagesurllib3connection.py", line 169, in _new_conn
conn = connection.create_connection(
File "C:Python39libsite-packagesurllib3utilconnection.py", line 73, in create_connection
for res in socket.getaddrinfo(host, port, family, socket.SOCK_STREAM):
File "C:Python39libsocket.py", line 953, in getaddrinfo
for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno 11001] getaddrinfo failed
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:Python39libsite-packagesurllib3connectionpool.py", line 699, in urlopen
httplib_response = self._make_request(
File "C:Python39libsite-packagesurllib3connectionpool.py", line 382, in _make_request
self._validate_conn(conn)
File "C:Python39libsite-packagesurllib3connectionpool.py", line 1010, in _validate_conn
conn.connect()
File "C:Python39libsite-packagesurllib3connection.py", line 353, in connect
conn = self._new_conn()
File "C:Python39libsite-packagesurllib3connection.py", line 181, in _new_conn
raise NewConnectionError(
urllib3.exceptions.NewConnectionError: <urllib3.connection.HTTPSConnection object at 0x000001413971AB50>: Failed to establish a new connection: [Errno 11001] getaddrinfo failed
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:Python39libsite-packagesrequestsadapters.py", line 439, in send
resp = conn.urlopen(
File "C:Python39libsite-packagesurllib3connectionpool.py", line 755, in urlopen
retries = retries.increment(
File "C:Python39libsite-packagesurllib3utilretry.py", line 574, in increment
raise MaxRetryError(_pool, url, error or ResponseError(cause))
urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='matic-mainnet-full-rpc.bwarelabs.com', port=443): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x000001413971AB50>: Failed to establish a new connection: [Errno 11001] getaddrinfo failed'))
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:textest.py", line 87, in get_tx_count
nonce = w3.eth.get_transaction_count(address)
File "C:Python39libsite-packagesweb3module.py", line 57, in caller
result = w3.manager.request_blocking(method_str,
File "C:Python39libsite-packagesweb3manager.py", line 186, in request_blocking
response = self._make_request(method, params)
File "C:Python39libsite-packagesweb3manager.py", line 147, in _make_request
return request_func(method, params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 76, in apply_formatters
response = make_request(method, params)
File "C:Python39libsite-packagesweb3middlewaregas_price_strategy.py", line 84, in middleware
return make_request(method, params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 74, in apply_formatters
response = make_request(method, formatted_params)
File "C:Python39libsite-packagesweb3middlewareattrdict.py", line 33, in middleware
response = make_request(method, params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 74, in apply_formatters
response = make_request(method, formatted_params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 76, in apply_formatters
response = make_request(method, params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 74, in apply_formatters
response = make_request(method, formatted_params)
File "C:Python39libsite-packagesweb3middlewarebuffered_gas_estimate.py", line 40, in middleware
return make_request(method, params)
File "C:Python39libsite-packagesweb3middlewareexception_retry_request.py", line 105, in middleware
return make_request(method, params)
File "C:Python39libsite-packagesweb3providersrpc.py", line 88, in make_request
raw_response = make_post_request(
File "C:Python39libsite-packagesweb3_utilsrequest.py", line 48, in make_post_request
response = session.post(endpoint_uri, data=data, *args, **kwargs) # type: ignore
File "C:Python39libsite-packagesrequestssessions.py", line 590, in post
return self.request('POST', url, data=data, json=json, **kwargs)
File "C:Python39libsite-packagesrequestssessions.py", line 542, in request
resp = self.send(prep, **send_kwargs)
File "C:Python39libsite-packagesrequestssessions.py", line 655, in send
r = adapter.send(request, **kwargs)
File "C:Python39libsite-packagesrequestsadapters.py", line 516, in send
raise ConnectionError(e, request=request)
requests.exceptions.ConnectionError: HTTPSConnectionPool(host='matic-mainnet-full-rpc.bwarelabs.com', port=443): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x000001413971AB50>: Failed to establish a new connection: [Errno 11001] getaddrinfo failed'))
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "C:textest.py", line 114, in main
print(get_tx_count(w3, address))
File "C:textest.py", line 90, in get_tx_count
raise NodeConnectionError(w3) from e
__main__.NodeConnectionError: An error occurred with the node at A web3 connection error occurred talking to https://matic-mainnet-full-rpc.bwarelabs.com:443..
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:textest.py", line 140, in <module>
main()
File "C:textest.py", line 116, in main
except NodeNotConnected(w3.provider.endpoint_uri, w3):
TypeError: catching classes that do not inherit from BaseException is not allowed
Test code:
from web3 import Web3
from time import sleep
from requests.exceptions import ConnectionError, ConnectTimeout, HTTPError
import sys
class NodeError(Exception):
"""Base exception for node errors"""
def __init__(self, url, msg=None):
if msg is None:
msg = f"An error occurred with the node at {url}."
super().__init__(msg)
self.url = url
class NodeNotConnected(NodeError):
"""web3 could not connect to a node"""
def __init__(self, url, w3=None):
msg = f"A web3 connection could not be made to URL {url}."
super().__init__(url, msg=msg)
self.w3 = w3
class NodeConnectionError(NodeError):
"""A web3 error occurred communicating with a node"""
def __init__(self, w3):
msg = (
f"A web3 connection error occurred talking to "
f"{w3.provider.endpoint_uri}."
)
super().__init__(msg)
self.w3 = w3
class NodeTooManyRequests(NodeError):
def __init__(self, w3):
msg = (
f"Too many requests made to {w3.provider.endpoint_uri}. Try a "
f"different node."
)
super().__init__(msg)
self.w3 = w3
class NodeNoAvailableNodes(NodeError):
def __init__(self):
msg = f"Unable to connect to any nodes."
super().__init__(msg)
class NodeInternalError(NodeError):
def __init__(self):
msg = "The node had an internal error."
super().__init__(msg)
def lib_connect_to_node(url):
"""Emulates library connect to node function"""
try:
w3 = Web3(
Web3.HTTPProvider(
url,
request_kwargs={"timeout": 5}
)
)
if not w3.isConnected():
raise NodeNotConnected(url, w3)
except Exception as e:
raise NodeNotConnected(url) from e
else:
return w3
def connect_to_node(urls, node_retries):
while node_retries >= 0:
try:
w3 = lib_connect_to_node(urls[0])
except NodeNotConnected(urls[0]) as e:
if node_retries == 0:
raise NodeNoAvailableNodes from e
node_retries -= 1
urls = get_next_node(urls)
print('Trying another node')
sleep(1)
continue
else:
return w3
def get_next_node(urls):
urls.append(urls.pop(urls.index(urls[0])))
return urls
def get_tx_count(w3, address):
try:
nonce = w3.eth.get_transaction_count(address)
except (ConnectionError, ConnectTimeout) as e:
print("A requests.exceptions.ConnectionError occurred.")
raise NodeConnectionError(w3) from e
except HTTPError as e:
if e.code == 429:
raise NodeTooManyRequests from e
else:
return nonce
def main():
urls = [
'https://matic-mainnet-full-rpc.bwarelabs.com:443',
'https://matic-mainnet.chainstacklabs.com:443',
'https://rpc-mainnet.maticvigil.com:443',
'https://rpc-mainnet.matic.network:443'
]
node_retries = 3
address = '0xe18A0D121057B002BaFb90aD5F1AB951594A61E8'
try:
w3 = connect_to_node(urls, node_retries)
except NodeNoAvailableNodes as e:
print(e)
sys.exit()
while True:
try:
print(get_tx_count(w3, address))
sleep(0.05)
except NodeNotConnected(w3.provider.endpoint_uri, w3):
urls = get_next_node(urls)
try:
w3 = connect_to_node(urls, node_retries)
except NodeNoAvailableNodes as e:
print(e)
sys.exit()
except NodeConnectionError(w3) as e:
urls = get_next_node(urls)
try:
w3 = connect_to_node(urls, node_retries)
except NodeNoAvailableNodes as e:
print(e)
sys.exit()
except NodeTooManyRequests(w3) as e:
print('Too many requests')
urls = get_next_node(urls)
try:
w3 = connect_to_node(urls, node_retries)
except NodeNoAvailableNodes as e:
print(e)
sys.exit()
if __name__ == '__main__':
main()
Answers:
You should be able to catch anything that is an exception, even if it doesn’t directly inherit from BaseException.
However, remove the class parameter from the except clause, and have only the class name:
try:
raise NodeNotConnected('abc')
except NodeNotConnected as e:
print(e)
Result:
A web3 connection could not be made to URL abc.
When you specify the class parameters, a class is instantiated, which evaluates to an instance instead of a type.
I’m trying to write some custom exception handling but keep running into issues with ‘TypeError: catching classes that do not inherit from BaseException is not allowed’ errors. I have a base exception class called NodeError that inherits from Exception. From there, I have several custom exceptions that inherit from NodeError.
The web3 module uses the requests module to communicate with a node. My test constantly tries to get the tx count from the node and while it does that, I try to simulate an outage by disabling my NIC. I try to catch requests.exceptions.ConnectionError in get_tx_count() and raise my own exception. It seems to correctly hit the NodeConnectionError custom exception based on the stack trace but then gets another exception and complains about catching classes that don’t inherit from BaseException.
Not sure why it thinks I’m not inheriting from BaseException but I have a feeling it has to do with catching the requests exception first.
Stack trace:
Traceback (most recent call last):
File "C:Python39libsite-packagesurllib3connection.py", line 169, in _new_conn
conn = connection.create_connection(
File "C:Python39libsite-packagesurllib3utilconnection.py", line 73, in create_connection
for res in socket.getaddrinfo(host, port, family, socket.SOCK_STREAM):
File "C:Python39libsocket.py", line 953, in getaddrinfo
for res in _socket.getaddrinfo(host, port, family, type, proto, flags):
socket.gaierror: [Errno 11001] getaddrinfo failed
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:Python39libsite-packagesurllib3connectionpool.py", line 699, in urlopen
httplib_response = self._make_request(
File "C:Python39libsite-packagesurllib3connectionpool.py", line 382, in _make_request
self._validate_conn(conn)
File "C:Python39libsite-packagesurllib3connectionpool.py", line 1010, in _validate_conn
conn.connect()
File "C:Python39libsite-packagesurllib3connection.py", line 353, in connect
conn = self._new_conn()
File "C:Python39libsite-packagesurllib3connection.py", line 181, in _new_conn
raise NewConnectionError(
urllib3.exceptions.NewConnectionError: <urllib3.connection.HTTPSConnection object at 0x000001413971AB50>: Failed to establish a new connection: [Errno 11001] getaddrinfo failed
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:Python39libsite-packagesrequestsadapters.py", line 439, in send
resp = conn.urlopen(
File "C:Python39libsite-packagesurllib3connectionpool.py", line 755, in urlopen
retries = retries.increment(
File "C:Python39libsite-packagesurllib3utilretry.py", line 574, in increment
raise MaxRetryError(_pool, url, error or ResponseError(cause))
urllib3.exceptions.MaxRetryError: HTTPSConnectionPool(host='matic-mainnet-full-rpc.bwarelabs.com', port=443): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x000001413971AB50>: Failed to establish a new connection: [Errno 11001] getaddrinfo failed'))
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:textest.py", line 87, in get_tx_count
nonce = w3.eth.get_transaction_count(address)
File "C:Python39libsite-packagesweb3module.py", line 57, in caller
result = w3.manager.request_blocking(method_str,
File "C:Python39libsite-packagesweb3manager.py", line 186, in request_blocking
response = self._make_request(method, params)
File "C:Python39libsite-packagesweb3manager.py", line 147, in _make_request
return request_func(method, params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 76, in apply_formatters
response = make_request(method, params)
File "C:Python39libsite-packagesweb3middlewaregas_price_strategy.py", line 84, in middleware
return make_request(method, params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 74, in apply_formatters
response = make_request(method, formatted_params)
File "C:Python39libsite-packagesweb3middlewareattrdict.py", line 33, in middleware
response = make_request(method, params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 74, in apply_formatters
response = make_request(method, formatted_params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 76, in apply_formatters
response = make_request(method, params)
File "cytoolz/functoolz.pyx", line 250, in cytoolz.functoolz.curry.__call__
return self.func(*args, **kwargs)
File "C:Python39libsite-packagesweb3middlewareformatting.py", line 74, in apply_formatters
response = make_request(method, formatted_params)
File "C:Python39libsite-packagesweb3middlewarebuffered_gas_estimate.py", line 40, in middleware
return make_request(method, params)
File "C:Python39libsite-packagesweb3middlewareexception_retry_request.py", line 105, in middleware
return make_request(method, params)
File "C:Python39libsite-packagesweb3providersrpc.py", line 88, in make_request
raw_response = make_post_request(
File "C:Python39libsite-packagesweb3_utilsrequest.py", line 48, in make_post_request
response = session.post(endpoint_uri, data=data, *args, **kwargs) # type: ignore
File "C:Python39libsite-packagesrequestssessions.py", line 590, in post
return self.request('POST', url, data=data, json=json, **kwargs)
File "C:Python39libsite-packagesrequestssessions.py", line 542, in request
resp = self.send(prep, **send_kwargs)
File "C:Python39libsite-packagesrequestssessions.py", line 655, in send
r = adapter.send(request, **kwargs)
File "C:Python39libsite-packagesrequestsadapters.py", line 516, in send
raise ConnectionError(e, request=request)
requests.exceptions.ConnectionError: HTTPSConnectionPool(host='matic-mainnet-full-rpc.bwarelabs.com', port=443): Max retries exceeded with url: / (Caused by NewConnectionError('<urllib3.connection.HTTPSConnection object at 0x000001413971AB50>: Failed to establish a new connection: [Errno 11001] getaddrinfo failed'))
The above exception was the direct cause of the following exception:
Traceback (most recent call last):
File "C:textest.py", line 114, in main
print(get_tx_count(w3, address))
File "C:textest.py", line 90, in get_tx_count
raise NodeConnectionError(w3) from e
__main__.NodeConnectionError: An error occurred with the node at A web3 connection error occurred talking to https://matic-mainnet-full-rpc.bwarelabs.com:443..
During handling of the above exception, another exception occurred:
Traceback (most recent call last):
File "C:textest.py", line 140, in <module>
main()
File "C:textest.py", line 116, in main
except NodeNotConnected(w3.provider.endpoint_uri, w3):
TypeError: catching classes that do not inherit from BaseException is not allowed
Test code:
from web3 import Web3
from time import sleep
from requests.exceptions import ConnectionError, ConnectTimeout, HTTPError
import sys
class NodeError(Exception):
"""Base exception for node errors"""
def __init__(self, url, msg=None):
if msg is None:
msg = f"An error occurred with the node at {url}."
super().__init__(msg)
self.url = url
class NodeNotConnected(NodeError):
"""web3 could not connect to a node"""
def __init__(self, url, w3=None):
msg = f"A web3 connection could not be made to URL {url}."
super().__init__(url, msg=msg)
self.w3 = w3
class NodeConnectionError(NodeError):
"""A web3 error occurred communicating with a node"""
def __init__(self, w3):
msg = (
f"A web3 connection error occurred talking to "
f"{w3.provider.endpoint_uri}."
)
super().__init__(msg)
self.w3 = w3
class NodeTooManyRequests(NodeError):
def __init__(self, w3):
msg = (
f"Too many requests made to {w3.provider.endpoint_uri}. Try a "
f"different node."
)
super().__init__(msg)
self.w3 = w3
class NodeNoAvailableNodes(NodeError):
def __init__(self):
msg = f"Unable to connect to any nodes."
super().__init__(msg)
class NodeInternalError(NodeError):
def __init__(self):
msg = "The node had an internal error."
super().__init__(msg)
def lib_connect_to_node(url):
"""Emulates library connect to node function"""
try:
w3 = Web3(
Web3.HTTPProvider(
url,
request_kwargs={"timeout": 5}
)
)
if not w3.isConnected():
raise NodeNotConnected(url, w3)
except Exception as e:
raise NodeNotConnected(url) from e
else:
return w3
def connect_to_node(urls, node_retries):
while node_retries >= 0:
try:
w3 = lib_connect_to_node(urls[0])
except NodeNotConnected(urls[0]) as e:
if node_retries == 0:
raise NodeNoAvailableNodes from e
node_retries -= 1
urls = get_next_node(urls)
print('Trying another node')
sleep(1)
continue
else:
return w3
def get_next_node(urls):
urls.append(urls.pop(urls.index(urls[0])))
return urls
def get_tx_count(w3, address):
try:
nonce = w3.eth.get_transaction_count(address)
except (ConnectionError, ConnectTimeout) as e:
print("A requests.exceptions.ConnectionError occurred.")
raise NodeConnectionError(w3) from e
except HTTPError as e:
if e.code == 429:
raise NodeTooManyRequests from e
else:
return nonce
def main():
urls = [
'https://matic-mainnet-full-rpc.bwarelabs.com:443',
'https://matic-mainnet.chainstacklabs.com:443',
'https://rpc-mainnet.maticvigil.com:443',
'https://rpc-mainnet.matic.network:443'
]
node_retries = 3
address = '0xe18A0D121057B002BaFb90aD5F1AB951594A61E8'
try:
w3 = connect_to_node(urls, node_retries)
except NodeNoAvailableNodes as e:
print(e)
sys.exit()
while True:
try:
print(get_tx_count(w3, address))
sleep(0.05)
except NodeNotConnected(w3.provider.endpoint_uri, w3):
urls = get_next_node(urls)
try:
w3 = connect_to_node(urls, node_retries)
except NodeNoAvailableNodes as e:
print(e)
sys.exit()
except NodeConnectionError(w3) as e:
urls = get_next_node(urls)
try:
w3 = connect_to_node(urls, node_retries)
except NodeNoAvailableNodes as e:
print(e)
sys.exit()
except NodeTooManyRequests(w3) as e:
print('Too many requests')
urls = get_next_node(urls)
try:
w3 = connect_to_node(urls, node_retries)
except NodeNoAvailableNodes as e:
print(e)
sys.exit()
if __name__ == '__main__':
main()
You should be able to catch anything that is an exception, even if it doesn’t directly inherit from BaseException.
However, remove the class parameter from the except clause, and have only the class name:
try:
raise NodeNotConnected('abc')
except NodeNotConnected as e:
print(e)
Result:
A web3 connection could not be made to URL abc.
When you specify the class parameters, a class is instantiated, which evaluates to an instance instead of a type.