Thursday, January 29, 2015

PySide Tree Tutorial IID: Adding helper methods to TreeItem

Part of a series on treebuilding in PySide: see Table of Contents.   

In part IIB we saw how to build a tree structure out of TreeItem instances. What we didn't discuss there is that each TreeItem comes packed with additional methods that will help us construct our tree model. Most of these methods let us extract simple data from a TreeItem, such as its parent, or the text from one of its columns. In this post, we'll go over these methods.

Via the child() method, an item returns one of its children from the given row:

def child(self, row):
    return self.childItems[row]

Recall from Figure 2 (post IB) that in tree models, children are arranged as a set of rows underneath their parents. Hence, to pick child n from a parent, just select the nth member from the parent's childItems list.

The number of children an item has is calculated in childCount():

def childCount(self):
    return len(self.childItems)

This just returns the length of the list that contains all the children.

The row() method reports an item's position among its siblings. For example, in Figure 5 (post IIB), the item marked A occupies row 0 among its siblings, while item D has the same parent, and occupies row 1. Item C is a child of A, and occupies row 1 among A's children.  

row() is implemented as follows:


def row(self):
    if self.parentItem:
        return self.parentItem.childItems.index(self) 
    return 0 

We should note a couple of things about row(). First, index() in this case has nothing to do with the model indexes mentioned in post IB: it is simply the built-in Python method that returns the index at which a given value occurs in a list. Second, our convention is that the root item is automatically assigned to row 0, even though it has no parent (Figure 5).


The number of columns of data associated with an item is returned by columnCount():

def columnCount(self):
    return len(self.itemData) 

Each item in our data structure contains the same number of columns, so we could have simply returned 2, but we have opted for a more general approach.

The data() method lets you extract the textual data from a particular column in a row (recall Figure 4, post IIB). It simply retrieves the text from the appropriate column stored in itemData:

def data(self, column):
    try:
        return self.itemData[column]
    except IndexError:
        return None

Note that data() returns None if the column index is out of range.

An item's parent is returned, surprisingly enough, from parent():


def parent(self):
    return self.parentItem

This will return None for the root node.

Summary
There you have it: we have built a beautiful tree of data, along with some methods that the model will use to extract different features of its nodes. What is interesting is that, up until now, we have officially done zero PySide programming. We have done all our work with standard Python classes, without importing QtGui, QtCore, or any other PySide classes. In the next section we will finally do some PySide programming, and look at how to wrap a model around this tree.

Tuesday, January 27, 2015

PySide Tree Tutorial IIC: Cross-examining simpletreemodel

Part of a series on treebuilding in PySide: see Table of Contents
 
In this tutorial, we are following simpletreemodel to the letter, simply trying to understand the code as given. But we should not assume that there is no room for improvement. For instance, in the code in post IIB, when initializing a TreeItem it would be helpful to add:

if parent is not None:
      parent.appendChild(self)

This would let us add a child to its parent immediately, right when the child is created. A significant simplification of the code. If you have other ways we might optimize the code, by all means let us know in the comments!

Further, if our goal was truly to build a simple, read-only model with a data store as small as the one that comes with this example, then we would weigh options other than QAbstractItemModel. In particular, we would want to consider using QTreeWidget or QStandardItemModel, which provide simplicity and ease-of-use in exchange for diminished speed and flexibility. In Part V, we will see how to create a tree like the one in our example using these tools. You'll see that the code is much simpler.

However, for larger models that require more flexibility and speed, you will probably end up subclassing QAbstractItemModel. In that case, simpletreemodel provides a useful template. While it was never meant to be a fully optimized industrial-grade example, it is helpful precisely because it provides a minimal working prototype to help people get their bearings, and contains little extraneous fluff.

Monday, January 26, 2015

PySide Tree Tutorial IIB: From TreeItem to tree structure

Part of a series on treebuilding in PySide: see Table of Contents

Recall from post IIA that the two classes we will focus on most are TreeItem and TreeModel. We'll start with TreeItem.

Each instance of TreeItem represents a node in our data tree, and we will bind multiple such items together into a hierarchical representation of all the data in our data set. As we will see, each node contains the text from one row of the model, as well as information about that node's relationship to other nodes in the tree.

The TreeItem class is defined as follows:

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class TreeItem(object):
    def __init__(self, data, parent=None):
        self.parentItem = parent
        self.itemData = data
        self.childItems = []

    def appendChild(self, item):
        self.childItems.append(item)

    def child(self, row):
        return self.childItems[row]

    def childCount(self):
        return len(self.childItems)

    def columnCount(self):
        return len(self.itemData)

    def data(self, column):
        try:
            return self.itemData[column]
        except IndexError:
            return None

    def parent(self):
        return self.parentItem

    def row(self):
        if self.parentItem:
            return self.parentItem.childItems.index(self)
        return 0

When instantiated, a TreeItem receives two inputs: data (a two-element list of strings) and  parent (which is typically another TreeItem):

def __init__(self, data, parent=None):
    self.parentItem = parent
    self.itemData = data
    self.childItems = []

Each TreeItem stores important data about its node. As illustrated in Figure 4, each node contains a full row of data, which consists of two columns (the title and summary). This columnar data is contained in itemData as a two-element list of strings.

Figure 4: Each TreeItem represents a row of the model
The diagram depicts how the title and summary information are stored in TreeItems A, B and C. Each TreeItem represents a row of data. The itemData attribute is a list that contains the data from each column. For instance, B.itemData[1] contains the string "Creating a GUI for your application."

In addition to storing an item's first-order data in itemData, each TreeItem also contains attributes that specify the item's place in the tree structure. Namely, it includes the item's parent (parent) as well as a list of the item's children (childItems). We can fully specify our tree's topography by appropriately setting these attributes for each node.

We can build our data tree using a simple and intuitive construction procedure. First, create the root TreeItem, which is the node with no parents. That is, the root item is the ancestor of all nodes in the tree, but a child of none:

rootItem = TreeItem(["Title", "Summary"], parent = None)

Note that the root's itemData contains the model's horizontal header data, but this is just for convenience: we could just as easily have sent it two empty strings. To attach children to the root item, we call TreeItem again, but with rootItem as the parent:

child0 = TreeItem(["Getting Started", "How to familiarize yourself..."], rootItem)
child1 = TreeItem(["Designing a component", "Creating a GUI for your app..."], rootItem)

Then, to add a child to one of these items, for instance to give child0 a child, we would follow the exact same procedure:


child00 = TreeItem(["Launching designer", "Running the Qt Designer app..."], child0)

Such iterative construction of TreeItems is all we need to generate our tree's basic skeleton, like the one illustrated in Figure 5.

Figure 5: The tree is built out of TreeItems.
TreeItems are connected together into a tree. As described in the text, we start with the root item, and proceed from there, instantiating new TreeItems with the root item as the parent, iterating this process until we have constructed the entire tree. Each item's row number, relative to its siblings, is indicated to the right.

There is one more step we must take to finish building our basic tree: we must fill in values for the childItems attribute. You probably noticed that when initialized, the childItems list started out empty. Once we have a population of TreeItems connected appropriately, we can use TreeItem.appendChild() to populate childItems:

def appendChild(self, item):
    self.childItems.append(item)

For instance, to add the children to the items defined above:

rootItem.appendChild(child0)
rootItem.appendChild(child1)
child0.appendChild(child00)

Once we have appended all the children to each item, then we are done building the tree! This tree structure serves as the data store with which our model will directly interact.

Note that the simpletreemodel example is a read-only model, so once the tree is grown it is frozen in place: its topography never changes. Hence, we will see (Part III) that appendChild() is only used in one place in our code: when initially constructing the tree with setupModelData(). Therein, creating a node and appending it to its parent are done in one line. For instance:

rootItem.appendChild(TreeItem(["Getting Started", "How to familiarize yourself..."], rootItem)

This line carries out the same operations as above, but in a more compact way.

If building a tree was our only goal, we would be done. But we want to display this tree using PySide. Since our model will need to provide certain methods for use by the view, we supplement each TreeItem instance with some additional methods that will help us build the model in Part III. We'll briefly look at each of these methods in Post IID.

But first, as an aside in Post IIC, we will briefly discuss other ways we might build a tree in PySide.

Friday, January 23, 2015

PySide Tree Tutorial IIA: An introduction to simpletreemodel

Part of a series on treebuilding in PySide: see Table of Contents

The simpletreemodel example that comes with PySide includes the following files:
simpletreemodel.py     #the main program
simpletreemodel_rc.py  #compiled resource file
default.txt            #text file of data in GUI
 The GUI displays a tree view of the Table of Contents of a Qt Designer tutorial (Figure 3). It provides basic keyboard navigation that is the default in all tree views (e.g., pressing the right arrow expands an item to show its children). Note that each row in the view contains two columns of data: a title (e.g.,'Getting Started') and a summary (e.g., 'How to familiarize yourself with Qt Designer').

Figure 3: The GUI created by the application

There are two main classes defined in simpletreemodel.py:
  1. A home-grown TreeItem class that represents individual rows in the tree. Our data store consists of multiple TreeItem instances connected into a hierarchically organized tree.
  2. A TreeModel class, subclassed from QAbstractItemModel, that serves as a wrapper for the data structure built out of TreeItems. It implements the API needed by the view (as discussed in post IA).
Parts II and III of this series will focus on these two classes, respectively.

Before moving on in the tutorial, I strongly recommend that you check to make sure that simpletreemodel runs as expected on your system. If it does not, then feel free to ask for help in the comments.

Tuesday, January 20, 2015

PySide Tree Tutorial IB: Models and Views -- The mighty index

Part of a series on treebuilding in PySide: see Table of Contents

One useful innovation in the development of the model/view framework is the use of indexes. Indexes are unique objects created by the model, with one index created for each separate item of data to be displayed by the view (e.g., each box in Figure 2). They act as wrappers that contain useful information about each individual data item.

The view uses these indexes to specify the location of the item about which it is making a query. This is analogous to using an index to pull a value from a simple Python list. However, as illustrated in Figure 2, uniquely specifying the location of an item in a data model is a bit more complicated. For instance, in a table model, specifying an item's location requires two numbers (the row and column of the item), and in a tree model three parameters are required (the row, column, and the index of the parent).


Figure 2: Indexes in tables and trees
A. Table model (rows and columns) Visual representation of a table model in which the location of each item is fixed by a row and a column number. We obtain the model index that refers to a data item by passing the relevant row and column numbers (and  a third optional parent parameter) to the model's index() method:

    indexA = model.index(0, 0, QtCore.QModelIndex())
    indexB = model.index(1, 1, QtCore.QModelIndex())
    indexC = model.index(2, 1, QtCore.QModelIndex())


Note that every item in the model (e.g., items A, B, and C) has the same parent. In general, the top level items in a model always have the invalid index QModelIndex() as their parent (see Part III). Since this is the default parent, for table models it is not required to send it explicitly.

B. Tree model (rows, columns, and parents) Visual representation of a tree model in which each item's location is fixed via a row number, a column number, and the index of its parent. Items A and C are top-level items in the model. Their indexes are obtained with:

    indexA = model.index(0, 0, QtCore.QModelIndex())
    indexC = model.index(2, 1, QtCore.QModelIndex())


Item A has multiple children, including item B. The index for item B is obtained with:

    indexB = model.index(1, 0, indexA)


We see that specifying the position of an item in a tree requires we indicate its parent index in addition to its row and column position.

Figure source:
http://qt-project.org/doc/qt-4.8/model-view-programming.html  

As we will see in Part III, model indexes are much more than bare coordinates. Each index includes multiple methods that supply additional useful information about the corresponding item in the model. For instance, index.internalPointer() returns the entire data item associated with index.

In general, indexes are one of the main mechanisms used to insulate views from raw data: they provide a universal format for the transmission of queries from the view to the model, and the view doesn't have to worry about the details of how the indexes were created.

We have purposely left open the question of how models interact with the data store. This is because data is messy, with no single universal format: you may end up using data from a SQL database, a text file, or even a simple Python list. If your data is in a SQL database and you want to display it as a table, then you will implement a QSqlTableModel. However, you will take a very different strategy if you want your data stored and displayed as a tree. In this case, the case dealt with in simpletreemodel, you will often subclass QAbstractItemModel (though see post IIC and part V).

Before this discussion gets too abstract to be helpful, let's take a break, and start looking at simpletreemodel in Part II.

Monday, January 19, 2015

PySide Tree Tutorial IA: Models and Views -- The Big Picture

Part of a series on treebuilding in PySide: see Table of Contents.

By separating content from appearance, the model/view framework takes a divide and conquer approach to GUI design. That is, the appearance of the data on the screen, handled by the view, is managed separately from the application's interactions with the data store. Such data wrangling is handled by the model.[1] This division of labor makes both of their jobs simpler: the view can ignore the details of how data are handled by the model, while the model can ignore the details of how data are painted to the screen.

As illustrated in Figure 1, views have two main roles:
  1.  Get data from the model by sending the model queries.
    Figure 1: The model/view framework
  2.    Paint the GUI to the screen: the same content can be viewed as a list, table, or a hierarchically organized tree. These different types of views are instances of QListView, QTableView, and QTreeView, respectively.[2]
Models also have two main roles:
  1. Handle all direct interactions with the data store, wrapping data into indexes to be used by the view.
  2.  Implement the expected interface with the view: when the view needs data, we know that the model will provide it using the mandatory interface.
While models and views are equally important, in these posts we will focus mainly on how to implement a model. The view class we will use is the factory-made QTreeView, whose default behavior will be fine for our purposes.

As mentioned above, all models are expected to instantiate a certain interface (or API) for the view to use. This API consists of a set of methods that provide views with all the information they need to paint the GUI to the screen. For instance, by invoking the model's data() method, we will see that the view can determine what text needs to be displayed on the screen. One nice feature of the standard views is that they will only request data about items in the model that presently need to be drawn to the screen.

The complexity of the model's API depends on the type of model you want to create. The simplest read-only list models only need to provide two methods: data() and rowCount().  More complex models, such as editable tree models, need to provide many additional methods. For a helpful overview of the methods required for different types of models, see Qt's Model subclassing reference (http://qt-project.org/doc/qt-4.8/model-view-programming.html#model-subclassing-reference).


[1] Note to keep things simple we will ignore the existence of delegates in this synopsis. Delegates control the display of individual data items and editors for said items, but for now we'll just lump this in as part of the view.

[2] For less standard visual representations of data (e.g., a bar graph or pie chart) you would need to make a custom view (Summerfield (2008), Chapter 16).

Saturday, January 17, 2015

PySide Tree Tutorial: Introduction

I think that I shall never see

A poem lovely as a tree.
                      -Joyce Kilmer


I've written an annotated companion to the simpletreemodel example that comes with PySide/PyQt. The code builds and displays a hierarchically organized data structure (i.e., a tree) using a model subclassed from QAbstractItemModel. It will take multiple posts (see Table of Contents, below, for publication schedule).

In Part I we will briefly review the model/view framework, focusing on details relevant for implementing our example. In sections II and III, we will study simpletreemodel. Part IV includes suggestions for future study. Part V shows how you can use other tools (QTreeWidget and QStandardItemModel) to construct a similar tree.

Table of Contents
I:Models and Views
            A. The big picture
            B. The mighty index
II: Constructing the data structure
            A. An introduction to simpletreemodel
            B. From TreeItem to tree structure
            C. Cross-examining simpletreemodel
III: Making the tree model
            A. Preparing for QAbstractItemModel (1/31)
            B. Common API (2/2)
            C. Implementing parent and index (2/4)
IV: Future directions (2/6)
V: Other routes to the tree
            A. QTreeWidget (2/8)
            B. QStandardItemModel (2/10)

Acknowledgments
This started as a direct translation of the online Qt documentation on model-view programming. The main sources used were:
Also, thanks also to the folks at stackoverflow and qtcentre.org for answering my many questions. Thanks also to Tim Doty, Anna Stenwick, and Mark Summerfield for comments on previous drafts. Feel free to post questions/suggestions/comments in the comments of the relevant posts, or email them to me: thomson.eric at gmail.

References
Summerfield, M (2008) Rapid Gui Programming with Python and Qt. Prentice Hall.
Summerfield, M (2010) Advanced Qt Programming. Prentice Hall.