用不同的元素遍历列表

Question

I have three types of element: a dict, a list with one dict, and a list with multiple dicts.

a = {"foo":1}
b = [{"foo":2}]
c = [{"foo":3}, {"foo":4}]
list = [a, b, c]

I want to be print all four values. The only ways I came up with were either checking type() of each element of list , or using try and except , like so:

for i in list:
    try:                     # i is a dict
        print(i["foo"])
    except:                  # i a list
        for e in i:
            print(e["foo"])

Is there any better way to do it?

Answer 1

You can use isinstance

Ex:

a = {"foo":1}
b = [{"foo":2}]
c = [{"foo":3}, {"foo":4}]
l = [a, b, c]

for i in l:
    if isinstance(i, dict):     #Check if dict object
        print(i["foo"])
    elif isinstance(i, list):   #Check if list object
        for j in i:
            print(j["foo"])

Output:

1
2
3
4

Answer 2

You can check the datatype explicitly:

mylist = [a, b, c]

for i in mylist:
    if isinstance(i,dict):
        print(i["foo"])
    elif isinstance(i,list):
        for e in i:
            print(e["foo"])

But this won't work if you call your variable list . This snippet illustrates why you should not do that.

Answer 3

You can use a recursive generator with your try / except idea:

a = {"foo":1}
b = [{"foo":2}]
c = [{"foo":3}, {"foo":4}]
L = [a, b, c]

def get_values(x):
    for i in x:
        try:
            yield i['foo']
        except TypeError:
            yield from get_values(i)

res = list(get_values(L))  # [1, 2, 3, 4]

Alternatively, using `isinstance:

def get_values(x):
    for i in x:
        if isinstance(i, dict):
            yield i['foo']
        else:
            yield from get_values(i)

Note you should never name variables after built-ins, eg use list_ or L instead of list .

Answer 4

If you want to do it for more nested elements like a list in a dict in a list you can create a function for every possible datatype.

class Printer():
    def print_element(self, element):
        if isinstance(element, dict):
            self._print_dict(element)
        elif isinstance(element, list):
            self._print_list(element)
        else:
            print(element)

    def _print_dict(self, element):
        for key,value in element.items():
            self.print_element(value)

    def _print_list(self, element):
        for value in element:
            self.print_element(value)

a = {"foo":1}
b = [{"foo":2}]
c = [{"foo":3}, {"foo":4}, [1, {"foo":[9,8,7,6]}]]

Printer().print_element(c)

Output:

3
4
1
9
8
7
6