I've been fighting with a weird bug in my app: if I'm sending a push notification to 2+ users, only the first one receives the notification.

The bug was that I .poped the message key from the data.

Here's how it works (pseudo-code):

def send(users: [User], data: dict):
    notifications = reduce(lambda acc, user: acc + [Notification(user, data)], users, [])
    db.bulk_insert(notifications)
    for notification in notifications:
        notification.send()

...

def Notification.send(self):
    message = self.data.pop('message') 
    Firebase.send(message, self.data)
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The first notification will be sent and I thought that message would be popped out of self.data, but actually it would be popped out of the dict that self.data points to! That means that every Notification object that has been initialized in the send function using Notification(user, data) (and not Notification(user, data.copy())) would not have data['message'] anymore, leading to unexpected behavior.

If you really need to get rid of a key in a dict, you should .copy() it before mutating.
If you don't - just stick to .get().