Home > Articles > Open Source > Python

This chapter is from the book

1.2. Builder Pattern

The Builder Pattern is similar to the Abstract Factory Pattern in that both patterns are designed for creating complex objects that are composed of other objects. What makes the Builder Pattern distinct is that the builder not only provides the methods for building a complex object, it also holds the representation of the entire complex object itself.

This pattern allows the same kind of compositionality as the Abstract Factory Pattern (i.e., complex objects are built out of one or more simpler objects), but is particularly suited to cases where the representation of the complex object needs to be kept separate from the composition algorithms.

We will show an example of the Builder Pattern in a program that can produce forms—either web forms using HTML, or GUI forms using Python and Tkinter. Both forms work visually and support text entry; however, their buttons are non-functional.* The forms are shown in Figure 1.2; the source code is in formbuilder.py.

Figure 1.2

Figure 1.2 The HTML and Tkinter forms on Windows

Let’s begin by looking at the code needed to build each form, starting with the top-level calls.

    htmlForm = create_login_form(HtmlFormBuilder())
    with open(htmlFilename, "w", encoding="utf-8") as file:

    tkForm = create_login_form(TkFormBuilder())
    with open(tkFilename, "w", encoding="utf-8") as file:

Here, we have created each form and written it out to an appropriate file. In both cases we use the same form creation function (create_login_form()), parameterized by an appropriate builder object.

def create_login_form(builder):
    builder.add_label("Username", 0, 0, target="username")
    builder.add_entry("username", 0, 1)
    builder.add_label("Password", 1, 0, target="password")
    builder.add_entry("password", 1, 1, kind="password")
    builder.add_button("Login", 2, 0)
    builder.add_button("Cancel", 2, 1)
    return builder.form()

This function can create any arbitrary HTML or Tkinter form—or any other kind of form for which we have a suitable builder. The builder.add_title() method is used to give the form a title. All the other methods are used to add a widget to the form at a given row and column position.

Both HtmlFormBuilder and TkFormBuilder inherit from an abstract base class, AbstractFormBuilder.

class AbstractFormBuilder(metaclass=abc.ABCMeta):

    def add_title(self, title):
        self.title = title

    def form(self):

    def add_label(self, text, row, column, **kwargs):

Any class that inherits this class must implement all the abstract methods. We have elided the add_entry() and add_button() abstract methods because, apart from their names, they are identical to the add_label() method. Incidentally, we are required to make the AbstractFormBuilder have a metaclass of abc.ABCMeta to allow it to use the abc module’s @abstractmethod decorator. (See §2.4, right-arrow.jpg 48 for more on decorators.)

Giving a class a metaclass of abc.ABCMeta means that the class cannot be instantiated, and so must be used as an abstract base class. This makes particular sense for code being ported from, say, C++ or Java, but does incur a tiny runtime overhead. However, many Python programmers use a more laid back approach: they don’t use a metaclass at all, and simply document that the class should be used as an abstract base class.

class HtmlFormBuilder(AbstractFormBuilder):

    def __init__(self):
        self.title = "HtmlFormBuilder"
        self.items = {}

    def add_title(self, title):

    def add_label(self, text, row, column, **kwargs):
        self.items[(row, column)] = ('<td><label for="{}">{}:</label></td>'
                .format(kwargs["target"], escape(text)))

    def add_entry(self, variable, row, column, **kwargs):
        html = """<td><input name="{}" type="{}" /></td>""".format(
                variable, kwargs.get("kind", "text"))
        self.items[(row, column)] = html

Here is the start of the HtmlFormBuilder class. We provide a default title in case the form is built without one. All the form’s widgets are stored in an items dictionary that uses row, column 2-tuple keys, and the widgets’ HTML as values.

We must reimplement the add_title() method since it is abstract, but since the abstract version has an implementation we can simply call that implementation. In this case we must preprocess the title using the html.escape() function (or the xml.sax.saxutil.escape() function in Python 3.2 or earlier).

The add_button() method (not shown) is structurally similar to the other add_...() methods.

    def form(self):
        html = ["<!doctype html>\n<html><head><title>{}</title></head>"
                "<body>".format(self.title), '<form><table border="0">']
        thisRow = None
        for key, value in sorted(self.items.items()):
            row, column = key
            if thisRow is None:
                html.append("  <tr>")
            elif thisRow != row:
                html.append("  </tr>\n  <tr>")
            thisRow = row
            html.append("    " + value)
        html.append("  </tr>\n</table></form></body></html>")
        return "\n".join(html)

The HtmlFormBuilder.form() method creates an HTML page consisting of a <form>, inside of which is a <table>, inside of which are rows and columns of widgets. Once all the pieces have been added to the html list, the list is returned as a single string (with newline separators to make it more human-readable).

class TkFormBuilder(AbstractFormBuilder):

    def __init__(self):
        self.title = "TkFormBuilder"
        self.statements = []

    def add_title(self, title):

    def add_label(self, text, row, column, **kwargs):
        name = self._canonicalize(text)
        create = """self.{}Label = ttk.Label(self, text="{}:")""".format(
                name, text)
        layout = """self.{}Label.grid(row={}, column={}, sticky=tk.W, \
padx="0.75m", pady="0.75m")""".format(name, row, column)
        self.statements.extend((create, layout))

    def form(self):
        return TkFormBuilder.TEMPLATE.format(title=self.title,
                name=self._canonicalize(self.title, False),
                statements="\n        ".join(self.statements))

This is an extract from the TkFormBuilder class. We store the form’s widgets as a list of statements (i.e., as strings of Python code), two statements per widget.

The add_label() method’s structure is also used by the add_entry() and add_button() methods (neither of which is shown). These methods begin by getting a canonicalized name for the widget and then make two strings: create, which has the code to create the widget and layout, which has the code to lay out the widget in the form. Finally, the methods add the two strings to the list of statements.

The form() method is very simple: it just returns a TEMPLATE string parameterized by the title and the statements.

    TEMPLATE = """#!/usr/bin/env python3
import tkinter as tk
import tkinter.ttk as ttk

class {name}Form(tk.Toplevel):  black-circle_1.jpg

    def __init__(self, master):
        self.withdraw()     # hide until ready to show
        self.title("{title}")  black-circle_2.jpg
        {statements}  black-circle_3.jpg
        self.bind("<Escape>", lambda *args: self.destroy())
        self.deiconify()    # show when widgets are created and laid out
        if self.winfo_viewable():

if __name__ == "__main__":
    application = tk.Tk()
    window = {name}Form(application)  black-circle_4.jpg
    application.protocol("WM_DELETE_WINDOW", application.quit)

The form is given a unique class name based on the title (e.g., LoginForm, black-circle_1.jpg; black-circle_4.jpg). The window title is set early on (e.g., “Login”, black-circle_2.jpg), and this is followed by all the statements to create and lay out the form’s widgets (black-circle_3.jpg).

The Python code produced by using the template can be run stand-alone thanks to the if __name__ ... block at the end.

    def _canonicalize(self, text, startLower=True):
        text = re.sub(r"\W+", "", text)
        if text[0].isdigit():
            return "_" + text
        return text if not startLower else text[0].lower() + text[1:]

The code for the _canonicalize() method is included for completeness. Incidentally, although it looks as if we create a fresh regex every time the function is called, in practice Python maintains a fairly large internal cache of compiled regexes, so for the second and subsequent calls, Python just looks up the regex rather than recompiling it.*

InformIT Promotional Mailings & Special Offers

I would like to receive exclusive offers and hear about products from InformIT and its family of brands. I can unsubscribe at any time.


Pearson Education, Inc., 221 River Street, Hoboken, New Jersey 07030, (Pearson) presents this site to provide information about products and services that can be purchased through this site.

This privacy notice provides an overview of our commitment to privacy and describes how we collect, protect, use and share personal information collected through this site. Please note that other Pearson websites and online products and services have their own separate privacy policies.

Collection and Use of Information

To conduct business and deliver products and services, Pearson collects and uses personal information in several ways in connection with this site, including:

Questions and Inquiries

For inquiries and questions, we collect the inquiry or question, together with name, contact details (email address, phone number and mailing address) and any other additional information voluntarily submitted to us through a Contact Us form or an email. We use this information to address the inquiry and respond to the question.

Online Store

For orders and purchases placed through our online store on this site, we collect order details, name, institution name and address (if applicable), email address, phone number, shipping and billing addresses, credit/debit card information, shipping options and any instructions. We use this information to complete transactions, fulfill orders, communicate with individuals placing orders or visiting the online store, and for related purposes.


Pearson may offer opportunities to provide feedback or participate in surveys, including surveys evaluating Pearson products, services or sites. Participation is voluntary. Pearson collects information requested in the survey questions and uses the information to evaluate, support, maintain and improve products, services or sites, develop new products and services, conduct educational research and for other purposes specified in the survey.

Contests and Drawings

Occasionally, we may sponsor a contest or drawing. Participation is optional. Pearson collects name, contact information and other information specified on the entry form for the contest or drawing to conduct the contest or drawing. Pearson may collect additional personal information from the winners of a contest or drawing in order to award the prize and for tax reporting purposes, as required by law.


If you have elected to receive email newsletters or promotional mailings and special offers but want to unsubscribe, simply email information@informit.com.

Service Announcements

On rare occasions it is necessary to send out a strictly service related announcement. For instance, if our service is temporarily suspended for maintenance we might send users an email. Generally, users may not opt-out of these communications, though they can deactivate their account information. However, these communications are not promotional in nature.

Customer Service

We communicate with users on a regular basis to provide requested services and in regard to issues relating to their account we reply via email or phone in accordance with the users' wishes when a user submits their information through our Contact Us form.

Other Collection and Use of Information

Application and System Logs

Pearson automatically collects log data to help ensure the delivery, availability and security of this site. Log data may include technical information about how a user or visitor connected to this site, such as browser type, type of computer/device, operating system, internet service provider and IP address. We use this information for support purposes and to monitor the health of the site, identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents and appropriately scale computing resources.

Web Analytics

Pearson may use third party web trend analytical services, including Google Analytics, to collect visitor information, such as IP addresses, browser types, referring pages, pages visited and time spent on a particular site. While these analytical services collect and report information on an anonymous basis, they may use cookies to gather web trend information. The information gathered may enable Pearson (but not the third party web trend services) to link information with application and system log data. Pearson uses this information for system administration and to identify problems, improve service, detect unauthorized access and fraudulent activity, prevent and respond to security incidents, appropriately scale computing resources and otherwise support and deliver this site and its services.

Cookies and Related Technologies

This site uses cookies and similar technologies to personalize content, measure traffic patterns, control security, track use and access of information on this site, and provide interest-based messages and advertising. Users can manage and block the use of cookies through their browser. Disabling or blocking certain cookies may limit the functionality of this site.

Do Not Track

This site currently does not respond to Do Not Track signals.


Pearson uses appropriate physical, administrative and technical security measures to protect personal information from unauthorized access, use and disclosure.


This site is not directed to children under the age of 13.


Pearson may send or direct marketing communications to users, provided that

  • Pearson will not use personal information collected or processed as a K-12 school service provider for the purpose of directed or targeted advertising.
  • Such marketing is consistent with applicable law and Pearson's legal obligations.
  • Pearson will not knowingly direct or send marketing communications to an individual who has expressed a preference not to receive marketing.
  • Where required by applicable law, express or implied consent to marketing exists and has not been withdrawn.

Pearson may provide personal information to a third party service provider on a restricted basis to provide marketing solely on behalf of Pearson or an affiliate or customer for whom Pearson is a service provider. Marketing preferences may be changed at any time.

Correcting/Updating Personal Information

If a user's personally identifiable information changes (such as your postal address or email address), we provide a way to correct or update that user's personal data provided to us. This can be done on the Account page. If a user no longer desires our service and desires to delete his or her account, please contact us at customer-service@informit.com and we will process the deletion of a user's account.


Users can always make an informed choice as to whether they should proceed with certain services offered by InformIT. If you choose to remove yourself from our mailing list(s) simply visit the following page and uncheck any communication you no longer want to receive: www.informit.com/u.aspx.

Sale of Personal Information

Pearson does not rent or sell personal information in exchange for any payment of money.

While Pearson does not sell personal information, as defined in Nevada law, Nevada residents may email a request for no sale of their personal information to NevadaDesignatedRequest@pearson.com.

Supplemental Privacy Statement for California Residents

California residents should read our Supplemental privacy statement for California residents in conjunction with this Privacy Notice. The Supplemental privacy statement for California residents explains Pearson's commitment to comply with California law and applies to personal information of California residents collected in connection with this site and the Services.

Sharing and Disclosure

Pearson may disclose personal information, as follows:

  • As required by law.
  • With the consent of the individual (or their parent, if the individual is a minor)
  • In response to a subpoena, court order or legal process, to the extent permitted or required by law
  • To protect the security and safety of individuals, data, assets and systems, consistent with applicable law
  • In connection the sale, joint venture or other transfer of some or all of its company or assets, subject to the provisions of this Privacy Notice
  • To investigate or address actual or suspected fraud or other illegal activities
  • To exercise its legal rights, including enforcement of the Terms of Use for this site or another contract
  • To affiliated Pearson companies and other companies and organizations who perform work for Pearson and are obligated to protect the privacy of personal information consistent with this Privacy Notice
  • To a school, organization, company or government agency, where Pearson collects or processes the personal information in a school setting or on behalf of such organization, company or government agency.


This web site contains links to other sites. Please be aware that we are not responsible for the privacy practices of such other sites. We encourage our users to be aware when they leave our site and to read the privacy statements of each and every web site that collects Personal Information. This privacy statement applies solely to information collected by this web site.

Requests and Contact

Please contact us about this Privacy Notice or if you have any requests or questions relating to the privacy of your personal information.

Changes to this Privacy Notice

We may revise this Privacy Notice through an updated posting. We will identify the effective date of the revision in the posting. Often, updates are made to provide greater clarity or to comply with changes in regulatory requirements. If the updates involve material changes to the collection, protection, use or disclosure of Personal Information, Pearson will provide notice of the change through a conspicuous notice on this site or other appropriate way. Continued use of the site after the effective date of a posted revision evidences acceptance. Please contact us if you have questions or concerns about the Privacy Notice or any objection to any revisions.

Last Update: November 17, 2020