Many places in prompt_toolkit require a boolean value that can change over time. For instance:

  • to specify whether a part of the layout needs to be visible or not;
  • or to decide whether a certain key binding needs to be active or not;
  • or the wrap_lines option of BufferControl;
  • etcetera.

These booleans are often dynamic and can change at runtime. For instance, the search toolbar should only be visible when the user is actually searching (when the search buffer has the focus). The wrap_lines option could be changed with a certain key binding. And that key binding could only work when the default buffer got the focus.

In prompt_toolkit, we decided to reduce the amount of state in the whole framework, and apply a simple kind of reactive programming to describe the flow of these booleans as expressions. (It’s one-way only: if a key binding needs to know whether it’s active or not, it can follow this flow by evaluating an expression.)

The (abstract) base class is Filter, which wraps an expression that takes no input and evaluates to a boolean. Getting the state of a filter is done by simply calling it.

An example

The most obvious way to create such a Filter instance is by creating a Condition instance from a function. For instance, the following condition will evaluate to True when the user is searching:

from prompt_toolkit.application.current import get_app
from prompt_toolkit.filters import Condition

is_searching = Condition(lambda: get_app().is_searching)

A different way of writing this, is by using the decorator syntax:

from prompt_toolkit.application.current import get_app
from prompt_toolkit.filters import Condition

def is_searching():
    return get_app().is_searching

This filter can then be used in a key binding, like in the following snippet:

from prompt_toolkit.key_binding import KeyBindings

kb = KeyBindings()

@kb.add('c-t', filter=is_searching)
def _(event):
    # Do, something, but only when searching.

If we want to know the boolean value of this filter, we have to call it like a function:


Built-in filters

There are many built-in filters, ready to use. All of them have a lowercase name, because they represent the wrapped function underneath, and can be called as a function.

  • has_arg
  • has_completions
  • has_focus
  • buffer_has_focus
  • has_selection
  • has_validation_error
  • is_aborting
  • is_done
  • is_read_only
  • is_multiline
  • renderer_height_is_known
  • in_editing_mode
  • in_paste_mode
  • vi_mode
  • vi_navigation_mode
  • vi_insert_mode
  • vi_insert_multiple_mode
  • vi_replace_mode
  • vi_selection_mode
  • vi_waiting_for_text_object_mode
  • vi_digraph_mode
  • emacs_mode
  • emacs_insert_mode
  • emacs_selection_mode
  • is_searching
  • control_is_searchable
  • vi_search_direction_reversed

Combining filters

Filters can be chained with the & (AND) and | (OR) operators and negated with the ~ (negation) operator.

Some examples:

from prompt_toolkit.key_binding import KeyBindings
from prompt_toolkit.filters import has_selection, has_selection

kb = KeyBindings()

@kb.add('c-t', filter=~is_searching)
def _(event):
    " Do something, but not while searching. "

@kb.add('c-t', filter=has_search | has_selection)
def _(event):
    " Do something, but only when searching or when there is a selection. "


Finally, in many situations you want your code to expose an API that is able to deal with both booleans as well as filters. For instance, when for most users a boolean works fine because they don’t need to change the value over time, while some advanced users want to be able this value to a certain setting or event that does changes over time.

In order to handle both use cases, there is a utility called to_filter().

This is a function that takes either a boolean or an actual Filter instance, and always returns a Filter.

from prompt_toolkit.filters.utils import to_filter

# In each of the following three examples, 'f' will be a `Filter`
# instance.
f = to_filter(True)
f = to_filter(False)
f = to_filter(Condition(lambda: True))
f = to_filter(has_search | has_selection)