feat: add default value for var (#549)

* feat: add default value for var

Signed-off-by: guowei shieh <guowei.shieh@outreach.io>

README.md

@@ -486,7 +486,7 @@ Available helpers and some examples:
 To be able to reuse a property from a teststep in a following testcase or step, you have to extract the variable, as the following example. 
 
 After the first step execution, `venom` extracts a value using a regular expression `foo with a ([a-z]+) here` from the content of the `result.systemout` property returned by the `executor`.
-Then this variable can be reused in another test, with the name `testA.myvariable` with `testA` corresponding to the name of the testcase.
+Then this variable can be reused in another test, with the name `testA.myvariable` with `testA` corresponding to the name of the testcase. A default value could also be supplied if the variable can't be extracted from the output, which can commonly happen when parsing json output.
 
 ```yaml
 name: MyTestSuite
@@ -499,6 +499,7 @@ testcases:
       myvariable:
         from: result.systemout
         regex: foo with a ([a-z]+) here
+        default: "somevalue"
 
 - name: testB
   steps:

process_testcase.go

@@ -432,9 +432,12 @@ func processVariableAssigments(ctx context.Context, tcName string, tcVars H, raw
 		if !has {
 			varValue, has = tcVars[tcName+"."+assigment.From]
 			if !has {
-				err := fmt.Errorf("%s reference not found in %s", assigment.From, strings.Join(tcVarsKeys, "\n"))
-				Info(ctx, "%v", err)
-				return nil, true, err
+				if assigment.Default == nil {
+					err := fmt.Errorf("%s reference not found in %s", assigment.From, strings.Join(tcVarsKeys, "\n"))
+					Info(ctx, "%v", err)
+					return nil, true, err
+				}
+				varValue = assigment.Default
 			}
 		}
 		if assigment.Regex == "" {

tests/exec.yml

@@ -33,4 +33,16 @@ testcases:
   - script: cat exec/testa.json
     info: "the value of result.systemoutjson is {{.result.systemoutjson}}"
     assertions:
-    - result.systemoutjson.foo ShouldContainSubstring bar
+    - result.systemoutjson.foo ShouldContainSubstring bar
+    vars:
+      foo:
+        from: result.systemoutjson.foo
+      bar:
+        from: result.systemoutjson.notexisting
+        default: "test"
+
+- name: verify default
+  steps:
+  - script: echo "{{.cat-json.foo}} {{.cat-json.bar}}"
+    assertions:
+    - result.systemout ShouldEqual "bar test"

types.go

@@ -250,8 +250,9 @@ type AssignStep struct {
 }
 
 type Assignment struct {
-	From  string `json:"from" yaml:"from"`
-	Regex string `json:"regex" yaml:"regex"`
+	From    string      `json:"from" yaml:"from"`
+	Regex   string      `json:"regex" yaml:"regex"`
+	Default interface{} `json:"default" yaml:"default"`
 }
 
 // RemoveNotPrintableChar removes not printable chararacter from a string