How do I use a keyword as a variable name?
I have the following class with the variables from
, to
and rate
. from
is a keyword. If I want to use it in the init method below, what's the correct way to write it?
More context: The class needs the from
variable explicitly as it's part of a json required by a POST endpoint written up by another developer in a different language. So changing the variable name is out of the question.
class ExchangeRates(JsonAware):
def __init__(self, from, to, rate):
self.from = from
self.to = to
self.rate = rate
JsonAware code:
class PropertyEquality(object):
def __eq__(self, other):
return (isinstance(other, self.__class__) and self.__dict__ == other.__dict__)
def __ne__(self, other):
return not self.__eq__(other)
def __repr__(self):
return '%s(%s)' % (self.__class__.__name__, ', '.join(['%s=%s' % (k, v) for (k, v) in self.__dict__.items()]))
class JsonAware(PropertyEquality):
def json(self):
return json.dumps(self, cls=GenericEncoder)
@classmethod
def from_json(cls, json):
return cls(**json)
GenericEncoder code:
class GenericEncoder(json.JSONEncoder):
def default(self, obj):
return obj.__dict__
Solution 1:
Add a single underscore to your preferred names: from_ and to_
(see PEP 8)
class ExchangeRates(JsonAware):
def __init__(self, from_, to_, rate):
self.from = from_
self.to = to_
self.rate = rate
Solution 2:
As mentioned in the comments, from
is a Python keyword so you can't use it as a variable name, or an attribute name. So you need to use an alternative name, and do a conversion when reading or writing the JSON data.
To do the output conversion you can supply a new encoder for json.dumps
; you can do that by overriding the ExchangeRates.json
method. To do the input conversion, override ExchangeRates.from_json
.
The strategy is similar in both cases: we create a copy of the dictionary (so we don't mutate the original), then we create a new key with the desired name and value, then delete the old key.
Here's a quick demo, tested on Python 2.6 and 3.6:
import json
class PropertyEquality(object):
def __eq__(self, other):
return (isinstance(other, self.__class__) and self.__dict__ == other.__dict__)
def __ne__(self, other):
return not self.__eq__(other)
def __repr__(self):
return '%s(%s)' % (self.__class__.__name__, ', '.join(['%s=%s' % (k, v) for (k, v) in self.__dict__.items()]))
class JsonAware(PropertyEquality):
def json(self):
return json.dumps(self, cls=GenericEncoder)
@classmethod
def from_json(cls, json):
return cls(**json)
class ExchangeRatesEncoder(json.JSONEncoder):
def default(self, obj):
d = obj.__dict__.copy()
d['from'] = d['frm']
del d['frm']
return d
class ExchangeRates(JsonAware):
def __init__(self, frm, to, rate):
self.frm = frm
self.to = to
self.rate = rate
def json(self):
return json.dumps(self, cls=ExchangeRatesEncoder)
@classmethod
def from_json(cls, json):
d = json.copy()
d['frm'] = d['from']
del d['from']
return cls(**d)
# Test
a = ExchangeRates('a', 'b', 1.23)
print(a.json())
jdict = {"from": "z", "to": "y", "rate": 4.56, }
b = ExchangeRates.from_json(jdict)
print(b.json())
typical output
{"from": "a", "to": "b", "rate": 1.23}
{"from": "z", "to": "y", "rate": 4.56}
Solution 3:
Use a synonym. Try "origin" or "source" instead.