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Python

What is a ConnectionError?

← Errors

A ConnectionError is raised in Python when a network operation fails to establish or maintain a connection. In the standard library it is a base class for network-related OSError subclasses. The requests library re-uses this name for HTTP connection failures.

Why it happens

  • •The target server is not running or is not reachable at the specified host and port.
  • •A firewall or network policy is blocking the connection.
  • •The URL has a typo, wrong port, or incorrect protocol (http vs https).
  • •The connection was established but dropped unexpectedly mid-request due to a timeout or server restart.

Examples

Python

Code that triggers the error

import requests

response = requests.get("http://localhost:8080/api/health")
print(response.status_code)

Error output

Traceback (most recent call last):
  ...
requests.exceptions.ConnectionError: HTTPConnectionPool(host='localhost', port=8080):
Max retries exceeded with url: /api/health
(Caused by NewConnectionError: Failed to establish a new connection:
[Errno 111] Connection refused)

How to fix it

Verify the target service is running and reachable (try curl or ping). Always set a timeout when making network requests to avoid hanging indefinitely. Wrap network calls in try/except and implement a retry strategy with exponential backoff for transient failures.

Fixed code

import requests
from requests.exceptions import ConnectionError as RequestsConnectionError

try:
    response = requests.get("http://localhost:8080/api/health", timeout=5)
    print(response.status_code)
except RequestsConnectionError:
    print("Service is unavailable. Check that it is running.")

Related errors

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