Neil's News

App Engine Python 3

31 January 2020

App Engine is a great platform for creating minimal cloud-hosted applications without fuss and bother. Here's a simple Python 2.7 application that takes two numbers as URL parameters, adds them, and prints them back out:

import cgi print("Content-Type: text/plain\n") forms = cgi.FieldStorage() a = 0 if "a" in forms: a = int(forms["a"].value) b = 0 if "b" in forms: b = int(forms["b"].value) print(a + b)

Thus when you visit /add.py?a=1&b=2 the result is 3.

Unfortunately App Engine's 'upgrade' from Python 2 to Python 3 appears to have made such lightweight applications impossible. First, app.yaml no longer provides routing of different URLs, so that logic needs to be moved into a singular Python program named main.py. Furthermore, the documentation states:

In the Python 3 runtime, your app needs to use a web framework such as Flask or Django

A quick glance at Flask (which describes itself as a "micro framework") reveals 300 kb of code. No.

Fortunately the documentation is a lie, it is perfectly possible to use Python 3 on App Engine without a framework:

import cgi def app(environ, start_response): # Route to requested handler. if environ["PATH_INFO"] == "/add.py": return add(environ, start_response) start_response("404 Not Found", []) return [b"Page not found."] def add(environ, start_response): forms = cgi.FieldStorage(fp=environ['wsgi.input'], environ=environ) a = 0 if "a" in forms: a = int(forms["a"].value) b = 0 if "b" in forms: b = int(forms["b"].value) out = str(a + b) headers = [ ("Content-Type", "text/plain") ] start_response("200 OK", headers) return [out.encode("utf-8")]

Hope this helps anyone else who is struggling to retain the simplicity of their Python 2 App Engine services during the forced upgrade to Python 3.

Update in 2024: The 'cgi' module is being removed from Python 3.13 with no replacement. Below is a solution for GET parameters.

from urllib.parse import unquote def app(environ, start_response): query_string = "" if "QUERY_STRING" in environ: query_string = environ["QUERY_STRING"] query_dict = parse_query(query_string) # Route to requested handler. if environ["PATH_INFO"] == "/add.py": return add(query_dict, start_response) start_response("404 Not Found", []) return [b"Page not found."] def add(query_dict, start_response): a = 0 if "a" in query_dict: a = int(query_dict["a"].value) b = 0 if "b" in query_dict: b = int(query_dict["b"].value) out = str(a + b) headers = [ ("Content-Type", "text/plain") ] start_response("200 OK", headers) return [out.encode("utf-8")] # Parse a query string (e.g. a=1&b=2) into a dictionary (e.g. {"a": 1, "b": 2}). # Very minimal parser. Does not combine repeated names (a=1&a=2), ignores # valueless names (a&b), does not support or isindex. def parse_query(query_string): parts = query_string.split('&') dict = {} for part in parts: tuple = part.split("=", 1) if len(tuple) == 2: dict[tuple[0]] = unquote(tuple[1]) return dict

For an example which handles POST data, see Glockenspiel's main.py.

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