如何最好地打印从用户定义的实例变量创建的字典？

Question

I am trying to organize my cows into a dictionary, access their values, and print them to the console.

Each instance of a cow is assigned to an index in list cow.

I am attempting to create a dictionary as follows:

for i in cows:
    cowDict[i.getName] = (i.getWeight, i.getAge)

I'd like to be able to access the values of my dictionary using my cows names, ie:

c["maggie"]

however, my code produces a key error.

If I print the entire dictionary, I get something to this effect:

"{<'bound method cow.getName of <'maggie, 3, 1>>: (<'bound method cow.getWeight of <'maggie, 3, 1>>, <'bound method cow.getAge of <'maggie, 3, 1>>), etc... }"

I can replace.getName with the instance variable and get the desired result, however, I've been advised away from that approach.

What is best practice to create a dictionary using instance variables of type cow?

Code:

class cow(object):
    """ A class that defines the instance objects name, weight and age associated
        with cows
    """
    def __init__(self, name, weight, age):
        self.name = name
        self.weight = weight
        self.age = age
    def getName(self):
        return self.name
    def getWeight(self):
        return self.weight
    def getAge(self):
        return self.age
    def __repr__(self):
        result = '<' + self.name + ', '  + str(self.weight) + ', ' + str(self.age) + '>'
        return result

def buildCows():
    """
    this function returns a dictionary of cows
    """
    names = ['maggie', 'mooderton', 'miles', 'mickey', 'steve', 'margret', 'steph']
    weights = [3,6,10,5,3,8,12]
    ages = [1,2,3,4,5,6,7]
    cows = []
    cowDict = {}
    for i in range(len(names)):
        #creates a list cow, where each index of list cow is an instance 
        #of the cow class
        cows.append(cow(names[i],weights[i],ages[i]))
    for i in cows:
        #creates a dictionary from the cow list with the cow name as the key
        #and the weight and age as values stored in a tuple
        cowDict[i.getName] = (i.getWeight, i.getAge)
    #returns a dictionary
    return cowDict

c = buildCows()

Answer 1

getName is a function so try

for i in cows:
    cowDict[i.getName()] = (i.getWeight(), i.getAge())

Answer 2

You can simplify this with something like:

names = ['maggie', 'mooderton', 'miles', 'mickey', 'steve', 'margret', 'steph']
weights = [3,6,10,5,3,8,12]
ages = [1,2,3,4,5,6,7]

cows = {names[i]: (weights[i], ages[i]) for i in range(len(names))}

for name in names:
    print '<%s, %d, %d>' % (name, cows[name][0], cows[name][1])

Which outputs:

<maggie, 3, 1>
<mooderton, 6, 2>
<miles, 10, 3>
<mickey, 5, 4>
<steve, 3, 5>
<margret, 8, 6>
<steph, 12, 7>

Answer 3

This looks like a job for zip . We can use it in conjunction with dict comprehensions to create dictionaries from arbitrary key and value expressions as follows

{name: (weight, age) for name, weight, age in zip(names, weights, ages)}`

We can then redefine buildCows as

def buildCows():
    names = ['maggie', 'mooderton', 'miles', 'mickey', 'steve', 'margret', 'steph']
    weights = [3,6,10,5,3,8,12]
    ages = [1,2,3,4,5,6,7]
    return {name: (weight, age) for name, weight, age in zip(names, weights, ages)}

Python has an open door policy on class attributes, so they are by default public unless, their names are preceded by underscores or double underscores (dunders). It is generally unnecessary to create getters/accessor methods, as you would in many other OO-languages. If you really do want private instance variables you can read up on them here

The reason for the erroneous results in the dict, was that it was being populated with references to the functions instead of calling them (ie i.getWeight should have been `i.getWeight()