Add parse priority

README.md

@@ -145,6 +145,12 @@ $ go run ./gopher.go
 age=44
 ```
 
+#### Parsing Priority 
+```go
+flag.ParsePriority = flag.ParsePriorityEnv // parse environment variables first (default)
+flag.ParsePriority = flag.ParsePriorityFile // parse configuration file first
+flag.Parse()
+```
 
 You can also parse prefixed environment variables by setting a prefix name when creating a new empty flag set:
 

extras.go

@@ -10,6 +10,11 @@ import (
 	"strings"
 )
 
+// Parse priority
+const ParsePriorityFile = "File"
+const ParsePriorityEnv = "Env"
+var ParsePriority = ParsePriorityEnv
+
 // EnvironmentPrefix defines a string that will be implicitely prefixed to a
 // flag name before looking it up in the environment variables.
 var EnvironmentPrefix = ""

flag.go

@@ -935,39 +935,52 @@ func (f *FlagSet) Parse(arguments []string) error {
 		}
 	}
 
-	// Parse environment variables
-	if err := f.ParseEnv(os.Environ()); err != nil {
+	// Prepare for parse configuration from file
+	var cFile string
+	if cf := f.formal[DefaultConfigFlagname]; cf != nil {
+		cFile = cf.Value.String()
+	}
+	if cf := f.actual[DefaultConfigFlagname]; cf != nil {
+		cFile = cf.Value.String()
+	}
+
+	// Parse environment variables and configuration from file
+	var envErr error
+	var fileErr error
+	if ParsePriority == ParsePriorityEnv {
+		envErr = f.ParseEnv(os.Environ())
+		if cFile != "" {
+			fileErr = f.ParseFile(cFile)
+		}
+	} else if ParsePriority == ParsePriorityFile {
+		if cFile != "" {
+			fileErr = f.ParseFile(cFile)
+		}
+		envErr = f.ParseEnv(os.Environ())
+	}
+
+	if envErr != nil {
 		switch f.errorHandling {
 		case ContinueOnError:
-			return err
+			return envErr
 		case ExitOnError:
 			os.Exit(2)
 		case PanicOnError:
-			panic(err)
+			panic(envErr)
 		}
-		return err
+		return envErr
 	}
 
-	// Parse configuration from file
-	var cFile string
-	if cf := f.formal[DefaultConfigFlagname]; cf != nil {
-		cFile = cf.Value.String()
-	}
-	if cf := f.actual[DefaultConfigFlagname]; cf != nil {
-		cFile = cf.Value.String()
-	}
-	if cFile != "" {
-		if err := f.ParseFile(cFile); err != nil {
-			switch f.errorHandling {
-			case ContinueOnError:
-				return err
-			case ExitOnError:
-				os.Exit(2)
-			case PanicOnError:
-				panic(err)
-			}
-			return err
+	if fileErr != nil {
+		switch f.errorHandling {
+		case ContinueOnError:
+			return fileErr
+		case ExitOnError:
+			os.Exit(2)
+		case PanicOnError:
+			panic(fileErr)
 		}
+		return fileErr
 	}
 
 	return nil