Mailing List Archive: Printing dict value for possibly undefined key

Printing dict value for possibly undefined key

Nov 24, 2023, 6:31 AM

Post #1 of 6 (134 views)

Hi,

I want to print some records from a database table where one of the
fields contains a JSON string which is read into a dict. I am doing
something like

print(f"{id} {d['foo']} {d['bar']}")

However, the dict does not always have the same keys, so d['foo'] or
d['bar'] may be undefined. I can obviously do something like

if not 'foo' in d:
d['foo']="NULL"
if not 'bar' in d:
d['bar']="NULL"
print(f"{id} {d['foo']} {d['bar']}")

Is there any more compact way of achieving the same thing?

Cheers,

Loris

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Re: Printing dict value for possibly undefined key [ In reply to ]

python-list at python

Nov 24, 2023, 8:35 AM

Post #2 of 6 (134 views)

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On 24/11/2023 14:31, Loris Bennett wrote:
> Hi,
>
> I want to print some records from a database table where one of the
> fields contains a JSON string which is read into a dict. I am doing
> something like
>
> print(f"{id} {d['foo']} {d['bar']}")
>
> However, the dict does not always have the same keys, so d['foo'] or
> d['bar'] may be undefined. I can obviously do something like
>
> if not 'foo' in d:
> d['foo']="NULL"
> if not 'bar' in d:
> d['bar']="NULL"
> print(f"{id} {d['foo']} {d['bar']}")
>
> Is there any more compact way of achieving the same thing?
>
> Cheers,
>
> Loris
>

Yes. e.g.

d.get('foo', "NULL")

Duncan
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Re: Printing dict value for possibly undefined key [ In reply to ]

python-list at python

Nov 24, 2023, 8:54 AM

Post #3 of 6 (134 views)

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On 24/11/2023 16:35, duncan smith wrote:
> On 24/11/2023 14:31, Loris Bennett wrote:
>> Hi,
>>
>> I want to print some records from a database table where one of the
>> fields contains a JSON string which is read into a dict. I am doing
>> something like
>>
>>    print(f"{id} {d['foo']} {d['bar']}")
>>
>> However, the dict does not always have the same keys, so d['foo'] or
>> d['bar'] may be undefined. I can obviously do something like
>>
>>    if not 'foo' in d:
>>      d['foo']="NULL"
>>    if not 'bar' in d:
>>      d['bar']="NULL"
>>    print(f"{id} {d['foo']} {d['bar']}")
>>
>> Is there any more compact way of achieving the same thing?
>>
>> Cheers,
>>
>> Loris
>>
>
> Yes. e.g.
>
> d.get('foo', "NULL")
>
> Duncan

Or make d a defaultdict.

from collections import defaultdict

dic = defaultdict(lambda:'NULL')
dic['foo'] = 'astring'
dic['foo']
'astring'
dic['bar']
'NULL'

Duncan
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Re: Printing dict value for possibly undefined key [ In reply to ]

python-list at python

Nov 25, 2023, 11:43 AM

Post #4 of 6 (134 views)

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On 11/25/2023 3:31 AM, Loris Bennett via Python-list wrote:
> Hi,
>
> I want to print some records from a database table where one of the
> fields contains a JSON string which is read into a dict. I am doing
> something like
>
> print(f"{id} {d['foo']} {d['bar']}")
>
> However, the dict does not always have the same keys, so d['foo'] or
> d['bar'] may be undefined. I can obviously do something like
>
> if not 'foo' in d:
> d['foo']="NULL"
> if not 'bar' in d:
> d['bar']="NULL"
> print(f"{id} {d['foo']} {d['bar']}")
>
> Is there any more compact way of achieving the same thing?

What does "the dict does not always have the same keys" mean?

a) there are two (or...) keys, but some records don't include both;

b) there may be keys other than 'foo' and 'bar' which not-known in-advance;

c) something else.

As mentioned, dict.get() solves one of these.

Otherwise, there are dict methods which collect/reveal all the keys, all
the values, or both - dict.keys(), .values(), .items(), resp.

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Re: Printing dict value for possibly undefined key [ In reply to ]

python-list at python

Nov 28, 2023, 5:58 AM

Post #5 of 6 (128 views)

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duncan smith <duncan@invalid.invalid> writes:

> On 24/11/2023 16:35, duncan smith wrote:
>> On 24/11/2023 14:31, Loris Bennett wrote:
>>> Hi,
>>>
>>> I want to print some records from a database table where one of the
>>> fields contains a JSON string which is read into a dict. I am doing
>>> something like
>>>
>>>    print(f"{id} {d['foo']} {d['bar']}")
>>>
>>> However, the dict does not always have the same keys, so d['foo'] or
>>> d['bar'] may be undefined. I can obviously do something like
>>>
>>>    if not 'foo' in d:
>>>      d['foo']="NULL"
>>>    if not 'bar' in d:
>>>      d['bar']="NULL"
>>>    print(f"{id} {d['foo']} {d['bar']}")
>>>
>>> Is there any more compact way of achieving the same thing?
>>>
>>> Cheers,
>>>
>>> Loris
>>>
>> Yes. e.g.
>> d.get('foo', "NULL")
>> Duncan
>
> Or make d a defaultdict.
>
> from collections import defaultdict
>
> dic = defaultdict(lambda:'NULL')
> dic['foo'] = 'astring'
> dic['foo']
> 'astring'
> dic['bar']
> 'NULL'
>
> Duncan
>

I have gone with the 'd.get' solution, as I am just need to print the
dict to the terminal. The dict is actually from a list of dicts which
is generated by querying a database, so I don't think the defaultdict
approach would be so appropriate, but it's good to know about it.

Thanks,

Loris
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Re: Printing dict value for possibly undefined key [ In reply to ]

python-list at python

Nov 28, 2023, 6:06 AM

Post #6 of 6 (128 views)

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DL Neil <PythonList@DancesWithMice.info> writes:

> On 11/25/2023 3:31 AM, Loris Bennett via Python-list wrote:
>> Hi,
>> I want to print some records from a database table where one of the
>> fields contains a JSON string which is read into a dict. I am doing
>> something like
>> print(f"{id} {d['foo']} {d['bar']}")
>> However, the dict does not always have the same keys, so d['foo'] or
>> d['bar'] may be undefined. I can obviously do something like
>> if not 'foo' in d:
>> d['foo']="NULL"
>> if not 'bar' in d:
>> d['bar']="NULL"
>> print(f"{id} {d['foo']} {d['bar']}")
>> Is there any more compact way of achieving the same thing?
>
>
> What does "the dict does not always have the same keys" mean?
>
> a) there are two (or...) keys, but some records don't include both;
>
> b) there may be keys other than 'foo' and 'bar' which not-known in-advance;
>
> c) something else.

Sorry for being unclear. There is either 'foo' or 'bar' or both, plus
some other keys which are always present.

> As mentioned, dict.get() solves one of these.
>
> Otherwise, there are dict methods which collect/reveal all the keys,
> all the values, or both - dict.keys(), .values(), .items(), resp.

That is a also a good point. I had forgotten about dict.keys() and
dict.values(), and hadn't been aware of dict.items().

Cheers,

Loris

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