Python Pandas从列表中的dict提取值

Question

Basically I would like to extract price value from this column, but the problem that if I use index [0] it would bring index[0] error in line 11 and price key also does not exit in this line.

ex
0    {u'availableToBack': [{u 'price' : 1.02, u 'size' : 2.15}], u 'availableToLay' : [], u 'tradedVolume' : []}
1    {u'availableToBack': [{u'price': 6.8, u'size':...
2    {u'availableToBack': [{u'price': 8.2, u'size':...
3    {u'availableToBack': [{u'price': 9.6, u'size':...
10   {u'availableToBack': [{u'price': 70.0, u'size'...
11   {u'availableToBack': [], u'availableToLay': []...
12   {u'availableToBack': [], u'availableToLay': []...

I use something like this

table['price'] = table['ex']['availableToBack'][0].apply(lambda x: x.get('price'))

Can you help me how to go around it, please?

JSON looks like this:

    u 'runners' : [{
            u 'status' : u 'ACTIVE',
            u 'handicap' : 0.0,
            u 'selectionId' : 10861647,
            u 'totalMatched' : 0.0,
            u 'adjustmentFactor' : 16.631,
            u 'ex' : {
                u 'availableToBack' : [{
                        u 'price' : 1.02,
                        u 'size' : 2.15
                    }
                ],
                u 'availableToLay' : [],
                u 'tradedVolume' : []
            }
        }, {
            u 'status' : u 'ACTIVE',
            u 'handicap' : 0.0,
            u 'selectionId' : 10861648,
            u 'totalMatched' : 0.0,
            u 'adjustmentFactor' : 13.237,
            u 'ex' : {
                u 'availableToBack' : [{
                        u 'price' : 1.01,
                        u 'size' : 7.11
                    }
                ],
                u 'availableToLay' : [],
                u 'tradedVolume' : []
            }
        },

Answer 1

10   {u'availableToBack': [{u'price': 70.0, u'size'...
11   {u'availableToBack': [], u'availableToLay': []...

I just saw that your entry for availableToBack contains an empty list. Put in a test which will set it to [dict()] if the list value is empty. This will allow the x.get('price', 0) to work correctly. Then you will be able to process the value of 'price' as 0 in the rest of your code.

You will then not have to test for 'None'.

x.get('price') has no "default" value specified so it returns "None". Set up a default value (such as x.get('price', 0) ) That way you should be able to perform the operations with a 0 price.

Python dictionary get() Method

Description

The method get() returns a value for the given key. If key is not available then returns default value None. Syntax

Following is the syntax for get() method −

dict.get(key, default=None)

Parameters

 key -- This is the Key to be searched in the dictionary. default -- This is the Value to be returned in case key does not exist. 

Return Value

This method returns a value for the given key. If key is not available, then it returns the default value None.

Answer 2

I don't think Python has a safe, built-in method for getting a value from a list without triggering an IndexError , but it is easy enough to write one. ( This answer has more detail on that.)

If I understand your intention correctly, this may help you. (Note that this only addresses the issue of getting the list item safely. You may want to use .get for each dictionary value you are retrieving for more safety.)

def safeget(seq, index, default=None):
    try:
        return seq[index]
    except IndexError:
        return default

for table in runners:
    price = safeget(table['ex']['availableToBack'], 0, default={}).get('price')
    print price

I tested against this structure, which has a an empty list for u'availableToBack in the second object.

runners = [
    {
        u'status' : u'ACTIVE',
        u'handicap' : 0.0,
        u'selectionId' : 10861647,
        u'totalMatched' : 0.0,
        u'adjustmentFactor' : 16.631,
        u'ex' : {
            u'availableToBack' : [
                    {u'price' : 1.02, u'size' : 2.15},
                ],
            u'availableToLay' : [],
            u'tradedVolume' : []
            }
    }, {
        u'status' : u'ACTIVE',
        u'handicap' : 0.0,
        u'selectionId' : 10861648,
        u'totalMatched' : 0.0,
        u'adjustmentFactor' : 13.237,
        u'ex' : {
            u'availableToBack' : [
                ## this one is MISSING a price
            ],
            u'availableToLay' : [],
            u'tradedVolume' : []
        }
    }
]

Answer 3

You can use a ternary operator. An empty list evaluates to false.

table['price'] = (table['ex']['availableToBack'][0].get('price') 
                  if table['ex']['availableToBack'] 
                  else None)

Answer 4

I found a solution and decided to post it. Basically maybe I did not explain well, but the problem is inconsistencies of having anything within the list.

table['availableToBack.price'] = table['ex'].apply(lambda x: x['availableToBack'][0].get('price') if (len(x['availableToBack']) != 0) else None)