A framework for providing optional arguments to functions.
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Updated Last
2 Years Ago
Started In
November 2012


The Options package provides macros that allow the simulation of optional keyword arguments for Julia functions. Some of this functionality was superseded by language support for keyword arguments (see #485). However, the Options package will likely remain relevant for cases where you need to pass optional arguments to nested functions without the parent functions needing to be aware of those arguments.


Install via the Julia package manager, Pkg.add("Options").

Basic usage

You gain access to Options via

using OptionsMod

The @defaults macro is for writing functions that take optional parameters. The typical syntax of such functions is:

  function myfunc(requiredarg1, requiredarg2, ..., opts::Options)
      @defaults opts a=11 b=2a+1 c=a*b d=100
      # The function body. Use a, b, c, and d just as you would
      # any other variable. For example,
      k = a + b
      # You can pass opts down to subfunctions, which might supply
      # additional defaults for other variables aa, bb, etc.
      y = subfun(k, opts)
      # Terminate your function with check_used, then return values
      @check_used opts
      return y

Note the function calls @check_used at the end; this will be discussed below.

It is possible to have more than one Options parameter to a function, for example:

  function twinopts(x, plotopts::Options, calcopts::Options)
      @defaults plotopts linewidth=1
      @defaults calcopts n_iter=100
      # Do stuff
      @check_used plotopts
      @check_used calcopts

Within a given scope, you should have only one call to @defaults per options variable.

The @options macro is typically the easiest way to set the value of optional parameters for a function that has been written to use them:

 opts = @options a=5 b=7

For a function that uses optional parameters a and b, this will override the default settings for these parameters. You would likely call that function in the following way:

 myfunc(requiredarg1, requiredarg2, ..., opts)

Argument checking

For functions that take many optional arguments, one fairly common problem is that the relevant options may change over time, but users may not notice the change and continue providing parameters that are no longer used by the function. A similar scenario arises when users simply misspell an optional parameter. In such cases the user may be quite confused about why changing the value of those parameters has no impact on the output of the function.

For this reason, the default expected behavior is for functions to check that all optional parameters have been used by the function. This is achieved with the @check_used macro. The test is performed at the end of the function body, so that subfunctions which handle parameters not used by the parent function may be "credited" for their usage. Each sub-function should also call @check_used, for example:

  function complexfun(x, opts::Options)
      @defaults opts parent=3 both=7
      subfun1(x, opts)
      subfun2(x, opts)
      @check_used opts
  function subfun1(x, opts::Options)
      @defaults opts sub1="sub1 default" both=0
      @check_used opts
  function subfun2(x, opts::Options)
      @defaults opts sub2="sub2 default" both=22
      @check_used opts

Typically, unused parameters will result in an error. The user can control the behavior this way:

 # throw an error if a or b is not used (the default)
 opts = @options CheckError a=5 b=2
 # issue a warning if a or b is not used
 opts = @options CheckWarn a=5 b=2
 # don't check whether a and b are used
 opts = @options CheckNone a=5 b=2

Additional functionality

As an alternative to the macro syntax, you can also say:

 opts = Options(CheckWarn, :a, 5, :b, 2)

The check flag is optional.

The @set_options macro lets you add new parameters to an existing options structure. For example:

  @set_options opts d=99

would add d to the set of parameters in opts, or re-set its value if it was already supplied.

Implementation details

The fields of the Options type are

  • key2index: a Dict that looks up an integer index, given the symbol for a variable (e.g., key2index[:a] for the variable a)
  • vals: vals[key2index[:a]] is the value to be assigned to the variable a
  • used: A vector of booleans, one per variable, with used[key2index[:a]] representing whether the value for variable a has been accessed by the @defaults macro
  • check_lock: A vector of booleans, one per variable. This is a "lock" that prevents sub-functions from complaining that they did not access variables that were intended for the parent function. @defaults sets the lock to true for any options variables that have already been defined; new variables added through @set_options will start with their check_lock set to false, to be handled by a subfunction.


The primary authors are Harlan Harris, Tim Holy, and Carlo Baldassi, with contributions from Stefan Karpinski, Patrick O'Leary, John Myles White, and Jeff Bezanson.