Widgets

Widget Layout

Urwid uses widgets to divide up the available screen space. This makes it easy to create a fluid interface that moves and changes with the user’s terminal and font size.

../_images/widget_layout.png

The result of rendering a widget is a canvas suitable for displaying on the screen. When we render the topmost widget:

  1. The topmost widget (a) is rendered the full size of the screen
  2. (a) renders (b) any size up to the full size of the screen
  3. (b) renders (c), (d) and (e) dividing its available screen columns between them
  4. (e) renders (f) and (g) dividing its available screen rows between them
  5. (e) combines the canvases from (f) and (g) and returns them
  6. (b) combines the canvases from (c), (d) and (e) and returns them
  7. (a) possibly modifies the canvas from (b) and returns it

Widgets (a), (b) and (e) are called container widgets because they contain other widgets. Container widgets choose the size and position their contained widgets.

Container widgets must also keep track of which one of their contained widgets is in focus. The focus is used when handling keyboard input. If in the above example (b) ‘s focus widget is (e) and (e) ‘s focus widget is (f) then keyboard input will be handled this way:

  1. The keypress is passed to the topmost widget (a)
  2. (a) passes the keypress to (b)
  3. (b) passes the keypress to (e), its focus widget
  4. (e) passes the keypress to (f), its focus widget
  5. (f) either handles the keypress or returns it
  6. (e) has an opportunity to handle the keypress if it was returned from (f)
  7. (b) has an opportunity to handle the keypress if it was returned from (e)
  8. (a) has an opportunity to handle the keypress if it was returned from (b)

Box, Flow and Fixed Widgets

The size of a widget is measured in screen columns and rows. Widgets that are given an exact number of screen columns and rows are called box widgets. The topmost widget is always a box widget.

Much of the information displayed in a console user interface is text and the best way to display text is to have it flow from one screen row to the next. Widgets like this that require a variable number of screen rows are called flow widgets. Flow widgets are given a number of screen columns and can calculate how many screen rows they need.

Occasionally it is also useful to have a widget that knows how many screen columns and rows it requires, regardless of the space available. This is called a fixed widget.

How a Widget’s Size is Determined
sizing mode width height
'box' container decides container decides
'flow' container decides widget’s rows() method
'fixed' widget’s pack() method widget’s pack() method

It is an Urwid convention to use the variables maxcol and maxrow to store a widget’s size. Box widgets require both of (maxcol, maxrow) to be specified.

Flow widgets expect a single-element tuple (maxcol,) instead because they calculate their maxrow based on the maxcol value.

Fixed widgets expect the value () to be passed in to functions that take a size because they know their maxcol and maxrow values.

Included Widgets

Widget class reference

../_images/urwid_widgets_1.png

Basic and graphic widgets are the content with which users interact. They may also be used as part of custom widgets you create.

../_images/urwid_widgets_2.png

Decoration Widgets

Decoration widgets alter the appearance or position of a single other widget. The widget they wrap is available as the original_widget property. If you might be using more than one decoration widget you may use the base_widget property to access the “most” original_widget. Widget.base_widget points to self on all non-decoration widgets, so it is safe to use in any situation.

Container Widgets

Container widgets divide their available space between their child widgets. This is how widget layouts are defined. When handling selectable widgets container widgets also keep track of which of their child widgets is in focus. Container widgets may be nested, so the actual widget in focus may be many levels below the topmost widget.

Urwid’s container widgets have a common API you can use, regardless of the container type. Backwards compatibility is still maintained for the old container-specific ways of accessing and modifying contents, but this API is now the preferred way of modifying and traversing containers.

container.focus

is a read-only property that returns the widget in focus for this container. Empty containers and non-container widgets (that inherit from Widget) return None.

container.focus_position

is a read/write property that provides access to the position of the container’s widget in focus. This will often be a integer value but may be any object. Columns, Pile, GridFlow, Overlay and ListBox with a SimpleListWalker or SimpleFocusListWalker as its body use integer positions. Frame uses 'body', 'header' and 'footer'; ListBox with a custom list walker will use the positions the list walker returns.

Reading this value on an empty container or on any non-container widgets (that inherit from Widget) raises an IndexError. Writing to this property with an invalid position will also raise an IndexError. Writing a new value automatically marks this widget to be redrawn and will be reflected in container.focus.

container.contents

is a read-only property (read/write in some cases) that provides access to a mapping- or list-like object that contains the child widgets and the options used for displaying those widgets in this container. The mapping- or list-like object always allows reading from positions with the usual __getitem__() method and may support assignment and deletion with __setitem__() and __delitem__() methods. The values are (child widget, option) tuples. When this object or its contents are modified the widget is automatically flagged to be redrawn.

Columns, Pile and GridFlow allow assigning an iterable to container.contents to overwrite the values in with the ones provided.

Columns, Pile, GridFlow, Overlay and Frame support container.contents item assignment and deletion.

container.options(...)

is a method that returns options objects for use in items added to container.contents. The arguments are specific to the container type, and generally match the __init__() arguments for the container. The objects returned are currently tuples of strings and integers or None for containers without child widget options. This method exists to allow future versions of Urwid to add new options to existing containers. Code that expects the option tuples to remain the same size will fail when new options are added, so defensive programming with options tuples is strongly encouraged.

container.__getitem__(x)
# a.k.a.
container[x]

is a short-cut method behaving identically to: container.contents[x][0].base_widget. Which means roughly “give me the child widget at position x and skip all the decoration widgets wrapping it”. Decoration widgets include Padding, Filler, AttrMap etc.

container.get_focus_path()

is a method that returns the focus position for this container and all child containers along the path defined by their focus settings. This list of positions is the closest thing we have to the singular widget-in-focus in other UI frameworks, because the ultimate widget in focus in Urwid depends on the focus setting of all its parent container widgets.

container.set_focus_path(p)

is a method that assigns to the focus_position property of each container along the path given by the list of positions p. It may be used to restore focus to a widget as returned by a previous call to container.get_focus_path().

container.get_focus_widgets()

is a method that returns the .focus values starting from this container and proceeding along each child widget until reaching a leaf (non-container) widget.

Note that the list does not contain the topmost container widget (i.e, on which this method is called), but does include the lowest leaf widget.

container.__iter__()
# typically
for x in container: ...

container.__reversed__()
# a.k.a
reversed(container)

are methods that allow iteration over the positions of this container. Normally the order of the positions generated by __reversed__() will be the opposite of __iter__(). The exception is the case of ListBox with certain custom list walkers, and the reason goes back to the original way list walker interface was defined. Note that a custom list walker might also generate an unbounded number of positions, so care should be used with this interface and ListBox.

Pile Widgets

Pile widgets are used to combine multiple widgets by stacking them vertically. A Pile can manage selectable widgets by keeping track of which widget is in focus and it can handle moving the focus between widgets when the user presses the UP and DOWN keys. A Pile will also work well when used within a ListBox.

A Pile is selectable only if its focus widget is selectable. If you create a Pile containing one Text widget and one Edit widget the Pile will choose the Edit widget as its default focus widget.

Columns Widgets

Columns widgets may be used to arrange either flow widgets or box widgets horizontally into columns. Columns widgets will manage selectable widgets by keeping track of which column is in focus and it can handle moving the focus between columns when the user presses the LEFT and RIGHT keys. Columns widgets also work well when used within a ListBox.

Columns widgets are selectable only if the column in focus is selectable. If a focus column is not specified the first selectable widget will be chosen as the focus column.

GridFlow Widgets

The GridFlow widget is a flow widget designed for use with Button, CheckBox and RadioButton widgets. It renders all the widgets it contains the same width and it arranges them from left to right and top to bottom.

The GridFlow widget uses Pile, Columns, Padding and Divider widgets to build a display widget that will handle the keyboard input and rendering. When the GridFlow widget is resized it regenerates the display widget to accommodate the new space.

Overlay Widgets

The Overlay widget is a box widget that contains two other box widgets. The bottom widget is rendered the full size of the Overlay widget and the top widget is placed on top, obscuring an area of the bottom widget. This widget can be used to create effects such as overlapping “windows” or pop-up menus.

The Overlay widget always treats the top widget as the one in focus. All keyboard input will be passed to the top widget.

If you want to use a flow flow widget for the top widget, first wrap the flow widget with a Filler widget.

ListBox Contents

ListBox is a box widget that contains flow widgets. Its contents are displayed stacked vertically, and the ListBox allows the user to scroll through its content. One of the flow widgets displayed in the ListBox is its focus widget.

ListBox Focus and Scrolling

The ListBox is a box widget that contains flow widgets. Its contents are displayed stacked vertically, and the ListBox allows the user to scroll through its content. One of the flow widgets displayed in the ListBox is the focus widget. The ListBox passes key presses to the focus widget to allow the user to interact with it. If the focus widget does not handle a keypress then the ListBox may handle the keypress by scrolling and/or selecting another widget to become the focus widget.

The ListBox tries to do the most sensible thing when scrolling and changing focus. When the widgets displayed are all Text widgets or other unselectable widgets then the ListBox will behave like a web browser does when the user presses UP, DOWN, PAGE UP and PAGE DOWN: new text is immediately scrolled in from the top or bottom. The ListBox chooses one of the visible widgets as its focus widget when scrolling. When scrolling up the ListBox chooses the topmost widget as the focus, and when scrolling down the ListBox chooses the bottommost widget as the focus.

The ListBox remembers the location of the widget in focus as either an “offset” or an “inset”. An offset is the number of rows between the top of the ListBox and the beginning of the focus widget. An offset of zero corresponds to a widget with its top aligned with the top of the ListBox. An inset is the fraction of rows of the focus widget that are “above” the top of the ListBox and not visible. The ListBox uses this method of remembering the focus widget location so that when the ListBox is resized the text displayed will stay roughly aligned with the top of the ListBox.

When there are selectable widgets in the ListBox the focus will move between the selectable widgets, skipping the unselectable widgets. The ListBox will try to scroll all the rows of a selectable widget into view so that the user can see the new focus widget in its entirety. This behavior can be used to bring more than a single widget into view by using composite widgets to combine a selectable widget with other widgets that should be displayed at the same time.

Dynamic ListBox with ListWalker

While the ListBox stores the location of its focus widget, it does not directly store the actual focus widget or other contents of the ListBox. The storage of a ListBox‘s content is delegated to a “List Walker” object. If a list of widgets is passed to the ListBox constructor then it creates a SimpleListWalker object to manage the list.

When the ListBox is rendering a canvas or handling input it will:

  1. Call the get_focus() method of its list walker object. This method will return the focus widget and a position object.
  2. Optionally call the get_prev() method of its List Walker object one or more times, initially passing the focus position and then passing the new position returned on each successive call. This method will return the widget and position object “above” the position passed.
  3. Optionally call the get_next() method of its List Walker object one or more times, similarly, to collect widgets and position objects “below” the focus position.
  4. Optionally call the set_focus() method passing one of the position objects returned in the previous steps.

This is the only way the ListBox accesses its contents, and it will not store copies of any of the widgets or position objects beyond the current rendering or input handling operation.

The SimpleListWalker stores a list of widgets, and uses integer indexes into this list as its position objects. It stores the focus position as an integer, so if you insert a widget into the list above the focus position then you need to remember to increment the focus position in the SimpleListWalker object or the contents of the ListBox will shift.

A custom List Walker object may be passed to the ListBox constructor instead of a plain list of widgets. List Walker objects must implement the List Walker Interface.

The fib.py example program demonstrates a custom list walker that doesn’t store any widgets. It uses a tuple of two successive Fibonacci numbers as its position objects and it generates Text widgets to display the numbers on the fly. The result is a ListBox that can scroll through an unending list of widgets.

The edit.py example program demonstrates a custom list walker that loads lines from a text file only as the user scrolls them into view. This allows even huge files to be opened almost instantly.

The browse.py example program demonstrates a custom list walker that uses a tuple of strings as position objects, one for the parent directory and one for the file selected. The widgets are cached in a separate class that is accessed using a dictionary indexed by parent directory names. This allows the directories to be read only as required. The custom list walker also allows directories to be hidden from view when they are “collapsed”.

Setting the Focus

The easiest way to change the current ListBox focus is to call the ListBox.set_focus() method. This method doesn’t require that you know the ListBox‘s current dimensions (maxcol, maxrow). It will wait until the next call to either keypress or render to complete setting the offset and inset values using the dimensions passed to that method.

The position object passed to set_focus() must be compatible with the List Walker object that the ListBox is using. For SimpleListWalker the position is the integer index of the widget within the list.

The coming_from parameter should be set if you know that the old position is “above” or “below” the previous position. When the ListBox completes setting the offset and inset values it tries to find the old widget among the visible widgets. If the old widget is still visible, if will try to avoid causing the ListBox contents to scroll up or down from its previous position. If the widget is not visible, then the ListBox will:

  • Display the new focus at the bottom of the ListBox if coming_from is “above”.
  • Display the new focus at the top of the ListBox if coming_from is “below”.
  • Display the new focus in the middle of the ListBox if coming_from is None.

If you know exactly where you want to display the new focus widget within the ListBox you may call ListBox.set_focus_valign(). This method lets you specify the top, bottom, middle, a relative position or the exact number of rows from the top or bottom of the ListBox.

List Walkers

ListBox does not manage the widgets it displays directly, instead it passes that task to a class called a “list walker”. List walkers keep track of the widget in focus and provide an opaque position object that the ListBox may use to iterate through widgets above and below the focus widget.

A SimpleFocusListWalker is a list walker that behaves like a normal Python list. It may be used any time you will be displaying a moderate number of widgets.

If you need to display a large number of widgets you should implement your own list walker that manages creating widgets as they are requested and destroying them later to avoid excessive memory use.

List walkers may also be used to display tree or other structures within a ListBox. A number of the example programs demonstrate the use of custom list walker classes.

List Walker Interface

List Walker API Version 1

This API will remain available and is still the least restrictive option for the programmer. Your class should subclass ListWalker. Whenever the focus or content changes you are responsible for calling ListWalker._modified().

MyV1ListWalker.get_focus()

return a (widget, position) tuple or (None, None) if empty

MyV1ListWalker.set_focus(position)

set the focus and call self._modified() or raise an IndexError.

MyV1ListWalker.get_next(position)

return the (widget, position) tuple below position passed or (None, None) if there is none.

MyV1ListWalker.get_prev(position)

return the (widget, position) tuple above position passed or (None, None) if there is none.

List Walker API Version 2

This API is an attempt to remove some of the duplicate code that V1 requires for many users. List walker API V1 will be implemented automatically by subclassing ListWalker and implementing the V2 methods. Whenever the focus or content changes you are responsible for calling ListWalker._modified().

MyV2ListWalker.__getitem__(position)

return widget at position or raise an IndexError or KeyError

MyV2ListWalker.next_position(position)

return the position below passed position or raise an IndexError or KeyError

MyV2ListWalker.prev_position(position)

return the position above passed position or raise an IndexError or KeyError

MyV2ListWalker.set_focus(position)

set the focus and call self._modified() or raise an IndexError.

MyV2ListWalker.focus

attribute or property containing the focus position, or define MyV1ListWalker.get_focus() as above

List Walker Iteration

There is an optional iteration helper method that may be defined in any list walker. When this is defined it will be used by ListBox.__iter__() and ListBox.__reversed__():

MyV2ListWalker.positions(reverse=False)

return a forward or reverse iterable of positions

Custom Widgets

Widgets in Urwid are easiest to create by extending other widgets. If you are making a new type of widget that can use other widgets to display its content, like a new type of button or control, then you should start by extending WidgetWrap and passing the display widget to its constructor.

The Widget interface is described in detail in the Widget base class reference and is useful if you’re looking to modify the behavior of an existing widget, build a new widget class from scratch or just want a better understanding of the library.

One Urwid design choice that stands out is that widgets typically have no size. Widgets don’t store their size on screen, and instead are passed that information when they need it.

This choice has some advantages:

  • widgets may be reused in different locations
  • reused widgets only need to be rendered once per size displayed
  • widgets don’t need to know their parents
  • less data to store and update
  • no worrying about widgets that haven’t received their size yet
  • same widgets could be displayed at different sizes to different users simultaneously

It also has disadvantages:

  • difficult to determine a widget’s size on screen
  • more parameters to parse
  • duplicated size calculations across methods

For determining a widget’s size on screen it is possible to look up the size(s) it was rendered at in the CanvasCache. There are plans to address some of the duplicated size handling code in the container widgets in a future Urwid release.

The same holds true for a widget’s focus state, so that too is passed in to functions that need it.

Modifying Existing Widgets

The easiest way to create a custom widget is to modify an existing widget. This can be done by either subclassing the original widget or by wrapping it. Subclassing is appropriate when you need to interact at a very low level with the original widget, such as if you are creating a custom edit widget with different behavior than the usual Edit widgets. If you are creating a custom widget that doesn’t need tight coupling with the original widget then wrapping is more appropriate.

The WidgetWrap class simplifies wrapping existing widgets. You can create a custom widget simply by creating a subclass of WidgetWrap and passing a widget into WidgetWrap’s constructor.

This is an example of a custom widget that uses WidgetWrap:

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import urwid

class QuestionnaireItem(urwid.WidgetWrap):
    def __init__(self):
        self.options = []
        unsure = urwid.RadioButton(self.options, u"Unsure")
        yes = urwid.RadioButton(self.options, u"Yes")
        no = urwid.RadioButton(self.options, u"No")
        display_widget = urwid.GridFlow([unsure, yes, no], 15, 3, 1, 'left')
        urwid.WidgetWrap.__init__(self, display_widget)

    def get_state(self):
        for o in self.options:
            if o.get_state() is True:
                return o.get_label()

The above code creates a group of RadioButtons and provides a method to query the state of the buttons.

Widgets from Scratch

Widgets must inherit from Widget. Box widgets must implement Widget.selectable() and Widget.render() methods, and flow widgets must implement Widget.selectable(), Widget.render() and Widget.rows() methods.

The default Widget.sizing() method returns a set of sizing modes supported from self._sizing, so we define _sizing attributes for our flow and box widgets below.

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import urwid

class Pudding(urwid.Widget):
    _sizing = frozenset(['flow'])

    def rows(self, size, focus=False):
        return 1

    def render(self, size, focus=False):
        (maxcol,) = size
        num_pudding = maxcol / len("Pudding")
        return urwid.TextCanvas(["Pudding" * num_pudding], maxcol=maxcol)


class BoxPudding(urwid.Widget):
    _sizing = frozenset(['box'])

    def render(self, size, focus=False):
        (maxcol, maxrow) = size
        num_pudding = maxcol / len("Pudding")
        return urwid.TextCanvas(["Pudding" * num_pudding] * maxrow,
                                maxcol=maxcol)

The above code implements two widget classes. Pudding is a flow widget and BoxPudding is a box widget. Pudding will render as much “Pudding” as will fit in a single row, and BoxPudding will render as much “Pudding” as will fit into the entire area given.

Note that the rows and render methods’ focus parameter must have a default value of False. Also note that for flow widgets the number of rows returned by the rows method must match the number of rows rendered by the render method.

To improve the efficiency of your Urwid application you should be careful of how long your rows() methods take to execute. The rows() methods may be called many times as part of input handling and rendering operations. If you are using a display widget that is time consuming to create you should consider caching it to reduce its impact on performance.

It is possible to create a widget that will behave as either a flow widget or box widget depending on what is required:

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import urwid

class MultiPudding(urwid.Widget):
    _sizing = frozenset(['flow', 'box'])

    def rows(self, size, focus=False):
        return 1

    def render(self, size, focus=False):
        if len(size) == 1:
            (maxcol,) = size
            maxrow = 1
        else:
            (maxcol, maxrow) = size
        num_pudding = maxcol / len("Pudding")
        return urwid.TextCanvas(["Pudding" * num_pudding] * maxrow,
                                maxcol=maxcol)

MultiPudding will work in place of either Pudding or BoxPudding above. The number of elements in the size tuple determines whether the containing widget is expecting a flow widget or a box widget.

Selectable Widgets

Selectable widgets such as Edit and Button widgets allow the user to interact with the application. A widget is selectable if its selectable method returns True. Selectable widgets must implement the Widget.keypress() method to handle keyboard input.

import urwid

class SelectablePudding(urwid.Widget):
    _sizing = frozenset(['flow'])
    _selectable = True

    def __init__(self):
        self.pudding = "pudding"

    def rows(self, size, focus=False):
        return 1

    def render(self, size, focus=False):
        (maxcol,) = size
        num_pudding = maxcol / len(self.pudding)
        pudding = self.pudding
        if focus:
            pudding = pudding.upper()
        return urwid.TextCanvas([pudding * num_pudding],
            maxcol=maxcol)

    def keypress(self, size, key):
        (maxcol,) = size
        if len(key) > 1:
            return key
        if key.lower() in self.pudding:
            # remove letter from pudding
            n = self.pudding.index(key.lower())
            self.pudding = self.pudding[:n] + self.pudding[n+1:]
            if not self.pudding:
                self.pudding = "pudding"
            self._invalidate()
        else:
            return key

The SelectablePudding widget will display its contents in uppercase when it is in focus, and it allows the user to “eat” the pudding by pressing each of the letters P, U, D, D, I, N and G on the keyboard. When the user has “eaten” all the pudding the widget will reset to its initial state.

Note that keys that are unhandled in the keypress method are returned so that another widget may be able to handle them. This is a good convention to follow unless you have a very good reason not to. In this case the UP and DOWN keys are returned so that if this widget is in a ListBox the ListBox will behave as the user expects and change the focus or scroll the ListBox.

Widget Displaying the Cursor

Widgets that display the cursor must implement the Widget.get_cursor_coords() method. Similar to the rows method for flow widgets, this method lets other widgets make layout decisions without rendering the entire widget. The ListBox widget in particular uses get_cursor_coords to make sure that the cursor is visible within its focus widget.

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import urwid

class CursorPudding(urwid.Widget):
    _sizing = frozenset(['flow'])
    _selectable = True

    def __init__(self):
        self.cursor_col = 0

    def rows(self, size, focus=False):
        return 1

    def render(self, size, focus=False):
        (maxcol,) = size
        num_pudding = maxcol / len("Pudding")
        cursor = None
        if focus:
            cursor = self.get_cursor_coords(size)
        return urwid.TextCanvas(["Pudding" * num_pudding], [], cursor, maxcol)

    def get_cursor_coords(self, size):
        (maxcol,) = size
        col = min(self.cursor_col, maxcol - 1)
        return col, 0

    def keypress(self, size, key):
        (maxcol, ) = size
        if key == 'left':
            col = self.cursor_col - 1
        elif key == 'right':
            col = self.cursor_col + 1
        else:
            return key
        self.cursor_x = max(0, min(maxcol - 1, col))
        self._invalidate()

CursorPudding will let the user move the cursor through the widget by pressing LEFT and RIGHT. The cursor must only be added to the canvas when the widget is in focus. The get_cursor_coords method must always return the same cursor coordinates that render does.

A widget displaying a cursor may choose to implement Widget.get_pref_col(). This method returns the preferred column for the cursor, and is called when the focus is moving up or down off this widget.

Another optional method is Widget.move_cursor_to_coords(). This method allows other widgets to try to position the cursor within this widget. The ListBox widget uses Widget.move_cursor_to_coords() when changing focus and when the user pressed PAGE UP or PAGE DOWN. This method must return True on success and False on failure. If the cursor may be placed at any position within the row specified (not only at the exact column specified) then this method must move the cursor to that position and return True.

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    def get_pref_col(self, (maxcol,)):
        return self.cursor_x

    def move_cursor_to_coords(self, (maxcol,), col, row):
        assert row == 0
        self.cursor_x = col
        return True

Widget Metaclass

The Widget base class has a metaclass defined that creates a __super attribute for calling your superclass: self.__super is the same as the usual super(MyClassName, self). This shortcut is of little use with Python 3’s new super() syntax, but will likely be retained for backwards compatibility in future versions.

This metaclass also uses MetaSignal to allow signals to be defined as a list of signal names in a signals class attribute. This is equivalent to calling register_signal() with the class name and list of signals and all those defined in superclasses after the class definition.