Question

Can someone explain what the dict class is used for? This snippet is from Dive Into Python

class FileInfo(dict):                  
    "store file metadata"
    def __init__(self, filename=None): 
        self["name"] = filename

I understand the assignment of key=value pairs with self['name'] = filename but what does inheriting the dict class have to do with this? Please help me understand.

Answer 1

It’s for creating your own customized Dictionary type.

You can override __init__, __getitem__ and __setitem__ methods for your own special purposes to extend dictionary’s usage.

Read the next section in the Dive into Python text: we use such inheritance to be able to work with file information just the way we do using a normal dictionary.

# From the example on the next section
>>> f = fileinfo.FileInfo("/music/_singles/kairo.mp3")
>>> f["name"]
'/music/_singles/kairo.mp3'

The fileinfo class is designed in a way that it receives a file name in its constructor, then lets the user get file information just the way you get the values from an ordinary dictionary.

Another usage of such a class is to create dictionaries which control their data. For example you want a dictionary who does a special thing when things are assigned to, or read from its ‘sensor’ key. You could define your special __setitem__ function which is sensitive with the key name:

def __setitem__(self, key, item):
    self.data[key] = item
    if key == "sensor":
        print("Sensor activated!")

Or for example you want to return a special value each time user reads the ‘temperature’ key. For this you subclass a __getitem__ function:

def __getitem__(self, key):
    if key == "temperature":
        return CurrentWeatherTemperature()
    else:
        return self.data[key]