Python：如何动态遍历dict对象？

Question

I want to write so data to a file, but the data can vary depending on its source.

At the moment my code just does the following:

try:
    file.write('{value1}'.format(value1=data["value1"]))
except:
    pass

try:
    file.write('{value2}'.format(value2=data["value2"]))
except:
    pass

#...

Due to the nature of the input data, these value keys may or may not be present, hence why I'm doing the above so when I get a KeyError I can just ignore it.

So my question is, is there a better way of doing the above? A more 'pythonic' way rather than this pretty wet solution?

example_input1:

'{"value1": 1.0, "value2": 1.2}'

example_input2:

'{"value2": 1.5}'

I guess ultimately I should standardise the input data, but unfortunately this is coming from a third-party API so isn't really an option.

Answer 1

Yes, you can simply use:

for key in ('value1','value2'):
    if key in data:
        file.write('{}'.format(data[key]))

So here if key in data first checks if there is a key in the dictionary (eg 'value1' ). Given that is the case, we use file.write('{}'.format(data[key])) to write the corresponding value to the file.

The advantage of using a for loop is furthermore that you can easily add and remove keys you want to print from the file.

If on the other hand you wish to write all keys that are in the dictionary, you can iterate over the dictionary :

for val in data.values():
    file.write('{}'.format(val))

Mind however that the order of the key/values in a dictionary is not necessary the insertion order. So the values can be shuffled.

Answer 2

You can use json library to convert your input string into dictionary-like object.

>>> import json
>>> input1 = '{"value1": 1.0, "value2": 1.2}'
>>> input2 = '{"value2": 1.5}'
>>> input1 = json.loads(input1)
>>> input2 = json.loads(input2)
>>> for key in input1.keys():
...  print(input1[key])
... 
1.2
1.0
>>> for key in input2.keys():
...  print(input2[key])
... 
1.5

Answer 3

If the order is not important in your case, you can just loop over your data like below:

for v in data.values():
    file.write('{}'.format(v))

Output:

>>> data = {"value1": 1.0, "value2": 1.2}
>>> for v in data.values():
...     print(v)
...
1.2
1.0
>>>

Answer 4

It depends, but I think you don't want to write all data you got, but only some one if they exists .

So here is a pythonic way ( try / except ):

inputs = [
    {"value1": "foo", "value2": "bar"},
    {"value1": "foo"},
    {"value3": "baz", 'unexpected': '??'}
]

expected_keys = ['value1', 'value2', 'value3']

for data in inputs:
    for key in expected_keys:
        try:
            file.write(data[key])
        except KeyError:
            pass

inputs is just a list of dict used as example, expected_keys is a list of keys you want to write if they exists in inputs.

With that you never miss an existing key and never raise KeyError if a key is missing.

This is the real pythonic way to handle this, see Python's EAFP