# Click Concepts

This section covers concepts about Click's design.

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(callback-evaluation-order)=

## Callback Evaluation Order

Click works a bit differently than some other command line parsers in that
it attempts to reconcile the order of arguments as defined by the
programmer with the order of arguments as defined by the user before
invoking any callbacks.

This is an important concept to understand when porting complex
patterns to Click from optparse or other systems.  A parameter
callback invocation in optparse happens as part of the parsing step,
whereas a callback invocation in Click happens after the parsing.

The main difference is that in optparse, callbacks are invoked with the raw
value as it happens, whereas a callback in Click is invoked after the
value has been fully converted.

Generally, the order of invocation is driven by the order in which the user
provides the arguments to the script; if there is an option called `--foo`
and an option called `--bar` and the user calls it as `--bar --foo`, then
the callback for `bar` will fire before the one for `foo`.

There are three exceptions to this rule which are important to know:

Eagerness:
>    An option can be set to be "eager".  All eager parameters are
>    evaluated before all non-eager parameters, but again in the order as
>    they were provided on the command line by the user.

>    This is important for parameters that execute and exit like `--help`
>    and `--version`.  Both are eager parameters, but whatever parameter
>    comes first on the command line will win and exit the program.

Repeated parameters:
>    If an option or argument is split up on the command line into multiple
>    places because it is repeated -- for instance, `--exclude foo --include
>    baz --exclude bar` -- the callback will fire based on the position of
>    the first option.  In this case, the callback will fire for
>    `exclude` and it will be passed both options (`foo` and
>    `bar`), then the callback for `include` will fire with `baz`
>    only.

>    Note that even if a parameter does not allow multiple versions, Click
>    will still accept the position of the first, but it will ignore every
>    value except the last.  The reason for this is to allow composability
>    through shell aliases that set defaults.

Missing parameters:
>    If a parameter is not defined on the command line, the callback will
>    still fire.  This is different from how it works in optparse where
>    undefined values do not fire the callback.  Missing parameters fire
>    their callbacks at the very end which makes it possible for them to
>    default to values from a parameter that came before.

Most of the time you do not need to be concerned about any of this,
but it is important to know how it works for some advanced cases.
