如何设置字典中是否存在var？

Question

I'm querying an API endpoint which brings information about events. The number of attributes about each event can vary though. I now try to do the following:

for e in apiResult:
    event = Event()
    event.street = e['f_street']
    event.price = e['f_price']
    event.venue_name = e['f_venue_name']
    event.start = e['f_start']
    # and about 40 more..
    event.save()

But I occasionally get key errors. So for each attribute I need to check whether it exists. I could of course do this as follows:

if 'f_street' in e:
    event.street = e['f_street']
if 'f_price' in e:
    event.price = e['f_price']
# etc. etc.

but this doesn't seem very Pythonic. Does anybody know how I can (Pythonically) set an attribute to event , only if it exists in the e dictionary? All tips are welcome!

Answer 1

I'd recommend to use list of attributes and for loop with if. Something like this:

    attrs = ['f_street', 'f_price', 'etc']
    ...
    for e in apiResult:
        event = Event()
        for attr in attrs:
            if attr in e:
                setattr(event, attr, e[attr])
        event.save()

This will also eliminate that 40 lines of attribute settings.

Btw, docs on python builtin setattr https://docs.python.org/2/library/functions.html#setattr

Answer 2

dict.get would help:

Help on built-in function get:

  get(...) method of builtins.dict instance  
    D.get(k[,d]) -> D[k] if k in D, else d.  d defaults to None.

So you could write like this:

# instead of this:
# event.start = e['f_start']
event.start = e.get('f_start') # And it would be None if key not exists

Answer 3

You can write a function for it:

def set_if_present(obj, attr_name, dictionary, key):
    if key in dictionary:
        setattr(obj, attr_name, dictionary[key])

Than you can use it like this:

>>> class O(object):
...     pass
... 
>>> d = {"a": 0, "b": 1}
>>> o = O()
>>> set_if_present(o, "a", d, "a")
>>> set_if_present(o, "b", d, "b")
>>> set_if_present(o, "c", d, "c")
>>> o.a
0
>>> o.b
1
>>> o.c
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
AttributeError: 'O' object has no attribute 'c'
>>> 

or in your case

for e in apiResult:
    event = Event()
    set_if_present(event, 'street', e, 'f_street')
    set_if_present(event, 'price', e, 'f_price')
    set_if_present(event, 'venue_name', e, 'f_venue_name')
    set_if_present(event, 'start', e, 'f_start')
    # and about 40 more..
    event.save()

To be even more pythonic, you can create a list of pairs like [('street', 'f_street'), ('price', 'f_price')...] and go through this one using the pairs to get and set your values.