详解Python Graphql
Posted gaojy
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前言
很高兴现在接手的项目让我接触到了Python Graphql,百度上对其介绍相对较少也不够全面,几乎没有完整的中文文档,所以这边也借此机会学习一下Graphql。
什么是Graphql呢?
Graphql是一个API查询语言,其数据由服务器上的一个Scheme提供,其查询返回的数据依赖请求的时候用户需要的精确数据。列如用户只需要一个name字段,服务器也只返回name的值。
参考
英文学习文档:https://graphene-python.org/
Hello Word 入门
先看下面一个例子,查询语句为{ hello(name:"gaojiayi") } 定义了要查询的入口,以及传入的参数。
from graphene import ObjectType, String, Schema class Query(ObjectType): """定义一个字符串属性域hello 且有一个字符串参数为name,设置name的默认""" hello = String(name = String(default_value="gaojy",required=True)) # resolve_hello定义了上面hello的实现,并返回查询结果 # 一般resolve需要加上固定前缀resolve_ @staticmethod def resolve_hello(root,info,name): return f"hello word -- {name}" schema = Schema(query=Query) if __name__ == ‘__main__‘: query_string = ‘‘‘{ hello(name:"gaojiayi") }‘‘‘ result = schema.execute(query_string) print(result.data[‘hello‘])
Graphql中的Types
Scheme
下面定义了一个Scheme,其中MyRootQuery,MyRootMutation,MyRootSubscription都是继承了graphene .objectType,但是不同之处在于query定义了查询数据的入口,而mutation用来数据改变或者数据恢复,而subscription是用来实时呈现数据的变化给client。type是用来指定返回数据的精确类型,列如返回的数据是一个interface,但是有多个类型继承了该interface,这时候需要指定一个具体的实现来返回给client。
my_schema = Schema( query=MyRootQuery, mutation=MyRootMutation, subscription=MyRootSubscription,
type=[SomeExtraObjectType,]
)
另外查询字符串默认为驼峰命名,列如
from graphene import ObjectType, String, Schema class Query(ObjectType): other_name = String(name=‘_other_Name‘) @staticmethod def resolve_other_name(root, info): return "test CamelCase" schema = Schema(query=Query) if __name__ == ‘__main__‘: # 查询数默认使用otherName,此处用了别名。 result = schema.execute(‘‘‘{_other_Name}‘‘‘) print(result.data[‘_other_Name‘])
如果关闭默认驼峰命名方式,则可以在定义scheme的时候加上auto_camelcase=False
my_schema = Schema(
auto_camelcase=False
)
scalars
scalars type可以理解为用来定义Field,它可以传入以下几种可选参数,例如
other_name = String(name=‘_other_Name‘,required=True,description="",deprecation_reason="",defalut_value=Any)
常见的基本saclars type有如下几个:
graphene.String
graphene.Int
graphene.Float
graphene.Boolean
graphene.ID
graphene.types.datetime.Date
graphene.types.datetime.DateTime
graphene.types.datetime.Time
graphene.types.json.JSONString
saclars type的挂载在objectType,interface,Mutation中的field域中。
class Person(graphene.ObjectType): name = graphene.String() # Is equivalent to: class Person(graphene.ObjectType): name = graphene.Field(graphene.String)
Lists and Non-Null
Non-Null
import graphene class Character(graphene.ObjectType): name = graphene.String(required=True) #等价于 即返回的数据如果name=null,则会报错 class Character(graphene.ObjectType): name = graphene.String(required=True)
Lists
import graphene class Character(graphene.ObjectType): # appears_in表示为一个非null元素的列表 appears_in = graphene.List(graphene.NonNull(graphene.String))
ObjectType
objectType是在scheme中用来定义Fields之间联系以及数据流转的python类,每一个obejctType属性表示一个Field,每个Field定义一个resolve方法用来获取数据,如果没有定义,则使用一个默认的resolver。
接下来看一个例子。
from graphene import ObjectType, String, Schema class Query(ObjectType): @staticmethod def resolve_hello(parent,info,name): return f"hello word -- {name}"
上面的resolve_hello有三个参数,分别是parent,info,name
1 parent通常用来获取objectType内的其他field的值,而在根query中默认为None,看下面的事例,当OjectType的Field为saclar type,则parent不会再向下传递。
class Person(ObjectType): full_name = String() def resolve_full_name(parent, info): return f"{parent.first_name} {parent.last_name}" class Query(ObjectType): me = Field(Person) def resolve_me(parent, info): # returns an object that represents a Person # 这里的parent为None return get_human(name="Luke Skywalker")
2 info表示请求的上下文,可以在查询语中添加context
3 name表示请求时带的参数,可以参考hello word事例,如有多个参数可形参**kwargs
from graphene import ObjectType, String class Query(ObjectType): hello = String(required=True, name=String()) def resolve_hello(parent, info, **kwargs): # name 为None 则name = World name = kwargs.get(‘name‘, ‘World‘) return f‘Hello, {name}!‘
4 默认resolver:列如一个objectType的field都没有指定队友的resolve,那么对象默认会序列化一个字典。
PersonValueObject = namedtuple(‘Person‘, ‘first_name‘, ‘last_name‘) class Person(ObjectType): first_name = String() last_name = String() class Query(ObjectType): me = Field(Person) my_best_friend = Field(Person) def resolve_me(parent, info): # always pass an object for `me` field # {"firstName": "Luke", "lastName": "Skywalker"} return PersonValueObject(first_name=‘Luke‘, last_name=‘Skywalker‘)
5 meta 类:用于objectType的配置
Enum
class Episode(graphene.Enum): NEWHOPE = 4 EMPIRE = 5 JEDI = 6 @property def description(self): if self == Episode.NEWHOPE: return ‘New Hope Episode‘ return ‘Other episode‘ class Query(ObjectType): desc1 = String( v=Argument(Episode, default_value=Episode.NEWHOPE.value), description=‘default value in schema is `4`, which is not valid. Also, awkward to write.‘) @staticmethod def resolve_desc1(parent, info,v): return f‘argument: {v!r}‘ # 使用下面的方式可以将python类型的enum转化成saclars类型 graphene.Enum.from_enum( AlreadyExistingPyEnum, description=lambda v: return ‘foo‘ if v == AlreadyExistingPyEnum.Foo else ‘bar‘)
Interfaces
顾名思义,接口,其他的obectType可以继承接口,示例如下
import graphene class Character(graphene.Interface): id = graphene.ID(required=True) name = graphene.String(required=True) friends = graphene.List(lambda: Character) #继承Character class Human(graphene.ObjectType): class Meta: interfaces = (Character, ) starships = graphene.List(Starship) home_planet = graphene.String() #继承Character class Droid(graphene.ObjectType): class Meta: interfaces = (Character, ) primary_function = graphene.String() class Query(graphene.ObjectType): # 返回的类型是Character hero = graphene.Field( Character, required=True, episode=graphene.Int(required=True) ) def resolve_hero(root, info, episode): # Luke is the hero of Episode V if episode == 5: return get_human(name=‘Luke Skywalker‘) return get_droid(name=‘R2-D2‘) #对于返回数据具体类型,可以在type属性中列举 schema = graphene.Schema(query=Query, types=[Human, Droid])
另外scheme中如果没有指定type,会报错
"Abstract type Character must resolve to an Object type at runtime for field Query.hero ..."
可以在interface中重写resolve_type方法
class Character(graphene.Interface): id = graphene.ID(required=True) name = graphene.String(required=True) #返回数据的时候,可以转换成具体的数据类型 @classmethod def resolve_type(cls, instance, info): if instance.type == ‘DROID‘: return Droid return Human
Union
该scalars type用来组合多个ObjectType,列如
import graphene class Human(graphene.ObjectType): name = graphene.String() born_in = graphene.String() class Droid(graphene.ObjectType): name = graphene.String() primary_function = graphene.String() class Starship(graphene.ObjectType): name = graphene.String() length = graphene.Int() # SearchResult组合了Human Droid Starship所有的Fields class SearchResult(graphene.Union): class Meta: types = (Human, Droid, Starship)
Mutations
如果说query是一个http get请求,那么Mutations可以看做是一个http post put请求。
def Mutate作为一个特殊的resover,当被调用的时候意在改变Mutation内的数据。
看下面一个操作示例
#具体的操作类 class CreatePerson(graphene.Mutation): # 请求提交的参数,同样需要传递到mutate中 class Arguments: name = graphene.String() ok = graphene.Boolean() person = graphene.Field(Person) def mutate(root, info, name): person = Person(name=name) ok = True #可执行具体的业务逻辑 包括写表 发消息等等 return CreatePerson(person=person, ok=ok) # Mutation class MyMutations(graphene.ObjectType): create_person = CreatePerson.Field() #指定mutation MyMutations schema = Schema(query=Query,mutation=MyMutations)
执行结果如下:
小结
技术本身就是为业务服务,读者会问Graphql究竟可以使用在哪些业务场景呢?
官方有这么一句话ask exactly what you want.如果一个前端的接口只需要返回部分数据,而另一个前端接口也只需要返回部分数据,这两份数据有可能有交集,也可能没有。传统的做法可能需要开发两个接口或者一个接口内不断的if else来根据前端的具体场景去过滤某些数据。使用Graphql能够根据client指定需要哪些参数,后端scheme返回哪些参数,而后端只需要一个API可以查询到数据全集,Graphql可以自动完成数据解析,封装,过滤操作。
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