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Overview ¶
Package flags provides an interface for automatically creating command line options from a struct.
Typically, if one wants to load from a yaml, one has to define a proper struct, then yaml.Unmarshal(), this is all good. However, there are situations where we want to load most of the configs from the file but overriding some configs.
Let's say we use a yaml to config our Db connections and upon start of the application we load from the yaml file to get the necessary parameters to create the connection. Our base.yaml looks like this
base.yaml --- mysql: user: 'foo' password: 'xxxxxx' mysql_defaults_file: ./mysql_defaults.ini mysql_socket_path: /var/run/mysqld/mysqld.sock ... more config options ...
we want to load all the configs from it but we want to provide some flexibility for the program to connect via a different db user. We could define a --user command flag then after loading the yaml file, we override the user field with what we get from --user flag.
If there are many overriding like this, manual define these flags is tedious. This package provides an automatic way to define this override, which is, given a struct, it'll create all the flags which are name using the field names of the struct. If one of these flags are set via command line, the struct will be modified in-place to reflect the value from command line, therefore the values of the fields in the struct are overridden
YAML is just used as an example here. In practice, one can use any struct to define flags.
Let's say we have our configration object as the following.
type logging struct {
Interval int
Path string
}
type socket struct {
ReadTimeout time.Duration
WriteTimeout time.Duration
}
type tcp struct {
ReadTimeout time.Duration
socket
}
type network struct {
ReadTimeout time.Duration
WriteTimeout time.Duration
tcp
}
type Cfg struct {
logging
network
}
The following code
func main() {
c := &Cfg{}
flags.ParseArgs(c, os.Args[1:])
}
will create the following flags
-logging.interval int
logging.interval
-logging.path string
logging.path
-network.readtimeout duration
network.readtimeout
-network.tcp.readtimeout duration
network.tcp.readtimeout
-network.tcp.socket.readtimeout duration
network.tcp.socket.readtimeout
-network.tcp.socket.writetimeout duration
network.tcp.socket.writetimeout
-network.writetimeout duration
network.writetimeout
flags to subcommands are naturally suported.
func main() {
cmd := os.Args[1]
switch cmd {
case "new"
c1 := &Cfg1{}
ParseArgs(c1, os.Args[2:])
case "update":
c2 := &Cfg2{}
ParseArgs(c2, os.Args[2:])
... more sub commands ...
}
}
One can set Flatten to true when calling NewFlagMakerAdv, in which case, flags are created without namespacing. For example,
type auth struct {
Token string
Tag float64
}
type credentials struct {
User string
Password string
auth
}
type database struct {
DBName string
TableName string
credentials
}
type Cfg struct {
logging
database
}
func main() {
c := &Cfg{}
flags.ParseArgs(c, os.Args[1:])
}
will create the following flags
-dbname string
dbname
-interval int
interval
-password string
password
-path string
path
-tablename string
tablename
-tag float
tag
-token string
token
-user string
user
Please be aware that usual GoLang flag creation rules apply, i.e., if there are duplication in flag names (in the flattened case it's more likely to happen unless the caller make due dilligence to create the struct properly), it panics.
Note that not all types can have command line flags created for. map, channel and function type will not defien a flag corresponding to the field. Pointer types are properly handled and slice type will create multi-value command line flags. That is, e.g. if a field foo's type is []int, one can use --foo 10 --foo 15 --foo 20 to override this field value to be []int{10, 15, 20}. For now, only []int, []string and []float64 are supported in this fashion.
Index ¶
Constants ¶
This section is empty.
Variables ¶
This section is empty.
Functions ¶
Types ¶
type FlagMaker ¶
type FlagMaker struct {
// contains filtered or unexported fields
}
FlagMaker enumerate all the exported fields of a struct recursively and create corresponding command line flags. For anonymous fields, they are only enumerated if they are pointers to structs. Usual GoLang flag rules apply, e.g. duplicated flag names leads to panic.
func NewFlagMaker ¶
func NewFlagMaker() *FlagMaker
NewFlagMaker creates a default FlagMaker which creates namespaced flags
func NewFlagMakerAdv ¶
func NewFlagMakerAdv(options *FlagMakingOptions) *FlagMaker
NewFlagMakerAdv gives full control to create flags.
func NewFlagMakerFlagSet ¶
func NewFlagMakerFlagSet(options *FlagMakingOptions, fs *flag.FlagSet) *FlagMaker
NewFlagMakerFlagSet gives full control to create flags.
func (*FlagMaker) PrintDefaults ¶
func (fm *FlagMaker) PrintDefaults()
PrintDefaults prints the default value and type of defined flags. It just calls the standard 'flag' package's PrintDefaults.
type FlagMakingOptions ¶
type FlagMakingOptions struct {
// Use lower case flag names rather than the field name/tag name directly.
UseLowerCase bool
// Create flags in namespaced fashion
Flatten bool
// If there is a struct tag named 'TagName', use its value as the flag name.
// The purpose is that, for yaml/json parsing we often have something like
// Foobar string `yaml:"host_name"`, in which case the flag will be named
// 'host_name' rather than 'foobar'.
TagName string
// If there is a struct tag named 'TagUsage', use its value as the usage description.
TagUsage string
}
FlagMakingOptions control the way FlagMaker's behavior when defining flags.
Source Files