每次用户单击按钮时如何更新字典中的键值？

Question

I can't update key-value in dictionary upon clicking the save button, but I don't know what's wrong with my code?

This is for my little project, I want to try writing code to create an app(for my personal use) using Tkinter to set and get data from entries and save to a csv file. But I've been stuck on updating my dictionary. I've tried making my dict a global variable, but it's still kept on deleting old key-value from the previous click. At some point during my trial-and-error, there's an error stating that my dict variable isn't defined.

def Save(self):

    self.new_first_name = self.first_name.get()
    self.new_last_name = self.last_name.get()
    self.new_id = self.id.get()
    self.new_age = self.age.get()

    self.dirname = "J:/Webcam"
    self.new_dict = {}

    def create_dict():
        global old_dict
        if 'old_dict' not in globals():
            global old_dict
            old_dict = {"ID": None, "First_Name" : None, "Last_Name": None, "Age": None}
        else:
            pass
        return old_dict
    create_dict()

    self.new_dict['ID'] = self.new_id
    self.new_dict['First_Name'] = self.new_first_name
    self.new_dict['Last_Name'] = self.new_last_name
    self.new_dict['Age'] = self.new_age

    old_dict.update(self.new_dict)

    # os.chdir(self.dirname)
    # w = csv.writer(open("output.csv", "w"))
    # for key, val in old_dict.items():
    #     w.writerow([key, val])
    return old_dict

This is the result after I clicked the 'save' button two times.

{'ID': 'AA', 'First_Name': 'AA', 'Last_Name': 'AA', 'Age': 'AA'}

{'ID': 'BB', 'First_Name': 'BB', 'Last_Name': 'BB', 'Age': 'BB'}

What I want is like this:

{'ID': ['AA','BB'], 'First_Name': ['AA','BB'], 'Last_Name': ['AA','BB'], 'Age': ['AA','BB']}

Answer 1

This problem is not specifically a TkInter problem. You can reproduce similar behavior with a python command-line.

Try this:

>>> old_dict = {}
>>> old_dict['ID'] = 'AA'
>>> old_dict['ID'] = 'BB'
>>> old_dict
{'ID': 'BB'}

That's because you first set the key 'ID' 's value to 'AA', and then to 'BB'.

What you want to do is make the dictionary contain a list for each key, and always append to those lists.

>>> old_dict = {}
>>> old_dict['ID'] = []
>>> old_dict['ID'].append('AA')
>>> old_dict['ID'].append('BB')
>>> old_dict
{'ID': ['AA', 'BB']}

You could save yourself a little effort by using collections.defaultdict :

>>> import collections
>>> old_dict = collections.defaultdict(list)
>>> old_dict['ID'].append('AA')
>>> old_dict['ID'].append('BB')
>>> old_dict
{'ID': ['AA', 'BB']}

All defaultdict does, the way this code is specified, is create a list for you for any key that doesn't already have a value.

If you apply this kind of logic to your code, you should find that tkInter is behaving the way you want it to.