jmespath 的使用，類似於jsonpath

在測試過程中，經常會取JSON中的某個值，jmespath可以是除了jsonpath的另外一種選擇.
下面通過幾個例子來說明jmespath在python的使用

jmespath python安裝

非常簡單直接pip,

pip install jmespath

查詢一個key值

source={"a": "foo", "b": "bar", "c": "baz"}
result = jmespath.search("a",source)
print(result)

這個的結果為：foo

subexpression

類似於jsonpath，通過.來表示路徑的層級

source_1={"a": {"b": {"c": {"d": "value"}}}}
sub_result = jmespath.search("a.b.c",source_1)
print(sub_result)

這個例子的結果為：{'d': 'value'}

index expressions

index expression主要使用在數組上

source_2 = ["a", "b", "c", "d", "e", "f"]
index_result = jmespath.search("[1]",source_2)
print(index_result)

這個例子的結果為:b

多個表達式綜合使用

以上幾種表達式可以合起來一期使用：

composite_exp = "a.b.c[0].d[1][0]"
source_3= {"a": {
 "b": {
 "c": [
 {"d": [0, [1, 2]]},
 {"d": [3, 4]}
 ]
 }
}}

composite_result = jmespath.search(composite_exp,source_3)
print(composite_result)

這個例子的結果為1

Slicing 切片

slicing 和python本身的slicing比較像，

source_4=[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
slicing_exp = "[0:5]"
slicing_result = jmespath.search(slicing_exp,source_4)
print(slicing_result)

這個例子的結果為： [0, 1, 2, 3, 4]

slicing實際上和python自己的機制基本一樣，同樣這個也是主要給數組使用.
有一點需要記住，基本的slicing的格式其實是： [start:stop:step]

Projections

projection不知道怎么翻譯，就先叫做投影吧，具體通過例子來說比較好理解.
projections主要包含一下幾種情況:

• List Projections
• Slice Projections
• Object Projections
• Flatten Projections
• Filter Projections

Projections- 例子

list_exp="people[*].first"
source_5 = {
 "people": [
 {"first": "James", "last": "d"},
 {"first": "Jacob", "last": "e"},
 {"first": "Jayden", "last": "f"},
 {"missing": "different"}
 ],
 "foo": {"bar": "baz"}
}

proj_result1= jmespath.search(list_exp,source_5)
print(proj_result1) # ['James', 'Jacob', 'Jayden']


obj_exp ="reservations[*].instances[*].state"
source_6= {
 "reservations": [
 {
 "instances": [
 {"state": "running"},
 {"state": "stopped"}
 ]
 },
 {
 "instances": [
 {"state": "terminated"},
 {"state": "runnning"}
 ]
 }
 ]
}

proj_result2=jmespath.search(obj_exp,source_6)
print(proj_result2) #[['running', 'stopped'], ['terminated', 'runnning']]

# Flatten projections

source_7=[
 [0, 1],
 2,
 [3],
 4,
 [5, [6, 7]]
]
flat_exp ="[]"
flat_result = jmespath.search(flat_exp,source_7)
print(flat_result) # [0, 1, 2, 3, 4, 5, [6, 7]]

# filter

filter_exp="machines[?state=='running'].name"
filter_source ={
 "machines": [
 {"name": "a", "state": "running"},
 {"name": "b", "state": "stopped"},
 {"name": "b", "state": "running"}
 ]
}
filter_result = jmespath.search(filter_exp,filter_source)
print(filter_result) # ['a','b']

# pipe expression

pipe_exp= "people[*].first | [0]"
pipe_source= {
 "people": [
 {"first": "James", "last": "d"},
 {"first": "Jacob", "last": "e"},
 {"first": "Jayden", "last": "f"},
 {"missing": "different"}
 ],
 "foo": {"bar": "baz"}
}

pipe_result = jmespath.search(pipe_exp,pipe_source)
print(pipe_result) # James

# multiselect
multi_exp="people[].[first,last]"
multiselect_result = jmespath.search(multi_exp,pipe_source)
print(multiselect_result) # [['James', 'd'], ['Jacob', 'e'], ['Jayden', 'f'], [None, None]]