Stop Python script from shutting down on small errors

Question

I have a Python socket server. In this socket server, when a new client connects and successfully authenticates themselves, it'll grab their json encoded information from the database. When it decodes it, it goes to a variable. Sometimes in the object, a key doesn't exist. I'll get a KeyError and the whole script will shut down. For example:

A client sends a packet to the server The server handles it like this:

def handleReceiveThisPacket(self, data):
    myInfo = self.info['exampleKey']
    return self.send(myInfo)

If "exampleKey" doesn't exist in the user's info (hey, it happens, like if I added a new key to the info for a new packet to handle something else and the user's info hasn't been updated yet), the whole script shuts down. I know I could easily do try, except but I often call keys throughout the script and I think it would look messy having a bunch of try and excepts everywhere, don't you think? Is there an easier way to just make it so Python gives off a warning about it but doesn't shut down, similar to how PHP does it?

Also, the client is a seperate class.

Answer 1

Use exception handling; to catch the exceptions; they exist for the sole reason that the normal program would be simpler, without a need for constant error checking. Also to get an optional key from dictionary you can use the get method

def handleReceiveThisPacket(self, data):
    # returns None if key not found
    myInfo = self.info.get('exampleKey')

    # returns 42 if key not found
    myInfo = self.info.get('exampleKey', 42)

    try:
        return self.send(myInfo)
    except Exception as e:
        print("Oops an exception occurred")
        print(e)

Answer 2

You can create your own MyDict class that wraps the KeyError and logs out warnings:

import logging

log = logging.getLogger(__name__)

class MyDict(dict):
    def __getitem__(self, key):
        try:
            return super(MyDict, self).__getitem__(key)
        except KeyError:
            log.warning('Attempted to get value for missing key %s.', str(key))
            return None

In [33]: d = MyDict()

In [34]: d['test']
Attempted to get value for missing key test.

Or in short (just for default values):

from collections import defaultdict

c = defaultdict(lambda: None)
c['test']

None

This example returns None as default value for missing keys, but you can customize it.

Answer 3

First of all, the PHP language made many bad design decisions, this is just one of them. Be happy that python detects and handles these problems in a nice way. I personally always wrote PHP code that never created any warnings. Once you have warnings it's hard to distinguish what's acceptable and what's a real problem.

Second, you can handle the problem several ways:

• You can handle these exceptions in an upper layer (1 or 2, instead of a dozen)
• You can use a defaultdict .
• You can use a utility function.

Example:

def getkey(d, k):
  try:
    return d[k]
  except KeyError:
    return ""

Note that all these solution have the serious flaw that they make your code brittle. You won't detect all the possible typos you can make for the keys.

I think the most clear way would be to fill your data set with values for the missing keys, so you don't have to do any error handling.

Answer 4

You may check if something is in a collection using the in operator:

>>> import json
>>> o = json.loads('{"a": 1}')
>>> "a" in o
True
>>> "b" in o
False

>>> o = dict(a=1)
>>> "a" in o
True
>>> "b" in o
False

>>> o = (1,2,3)
>>> 1 in o
True
>>> 4 in o
False

Also, dict s have a .get(key, default=None) method .

Answer 5

Catch the exception. This link explains exception handling in Python. Put a try... except KeyError... around the relevant code.

Alternatively, use the dictionary method get. myInfo = self.info.get('exampleKey', default_value) won't raise a KeyError, and will return default_value if 'exampleKey' isn't in myInfo.

A third option would be to check if the key is in the dictionary first: if 'exampleKey' in self.info: myInfo = self.info['exampleKey'] .