Booleano: Boolean Expressions Interpreter

Author:Gustavo Narea.
Latest release:1.0a1
Date:December 21, 2009.


Booleano is an interpreter of boolean expressions, a library to define and run filters available as text (e.g., in a natural language) or in Python code.

In order to handle text-based filters, Booleano ships with a fully-featured parser whose grammar is adaptive: Its properties can be overridden using simple configuration directives.

On the other hand, the library exposes a pythonic API for filters written in pure Python. These filters are particularly useful to build reusable conditions from objects provided by a third party library.

The Three Use Cases

Booleano has been designed to address the following use cases:

  1. Convert text-based conditions: When you need to turn a condition available as plain text into something else (i.e., another filter).
  2. Evaluate text-based conditions: When you have a condition available as plain text and need to iterate over items in order to filter out those for which the evaluation of the condition is not successful.
  3. Evaluate Python-based conditions: When you have a condition represented by a Python object (nothing to be parsed) and need to iterate over items in order to filter out those for which the evaluation of the condition is not successful.

Convert text-based conditions

Say you have an online bookstore and you want your book search engine to support advanced queries that are hard to create with forms. You can have Booleano convert your users’ query expressions into safe SQL WHERE clauses.

For example, if an user enters:

"python" in title and category == "computing" and (rating > 3 or publication_date:year => 2007) and (not software or software:works_on_linux)

Booleano will parse that expression and will use a converter (defined by you) to turn the resulting parse tree into a WHERE clause which filters the books that meet all the requirements below:

  • The book title contains the word “python”.
  • The book falls into the category “computing”.
  • The book has an average rating greater than 3 or it was published after 2006.
  • If the books ships with software (e.g., in a CD), the software must work under Linux.

Of course, Booleano could also handle a simpler expression.


The conversion result doesn’t have to be text too, it can be any Python object. For example, if you use SQLAlchemy, your converter can turn the parse trees into SQLAlchemy filters.

Evaluate text-based conditions

Say you’ve created an alternative to the Unix utility find, but unlike find, users of your application don’t filter results with command switches. Instead, they use boolean expressions to filter the files/directories.

For example, if an user runs the following command (where “search” is the name of your application):

search / 'file:extension in {"html", "htm", "xhtml"} and ("www-data" in file:owner:groups or users:current_user == file:owner)'

Then, Booleano will parse the expression delimited by single quotes and search will iterate over all the files in the filesystem. On every iteration, search will use the parse tree returned by Booleano and will evaluate it against the current file; if evaluation fails, the file is excluded from the results; otherwise, it’s included.

With the fictitious command above, only those HTML documents that meet at least one of the following conditions are included:

  • The owner of the file is a member of the group “www-data”.
  • The owner of the file is the user that is running the command.

Again, Booleano could also handle a simpler expression (such as 'file:type == mime_types:html' just to filter in all the HTML documents).

Evaluate Python-based conditions

Say you’re using a third party authorization library which grants access if and only if the callable you pass to it returns True. On the other hand, the library provides you with one Booleano variable (which is an stateless Python object) called “current_user”, that represents the name of the current user.

You could build your Python-based condition like this:

>>> from authorization_library import current_user, set_condition
>>> condition = current_user == "foo"
>>> condition
<Equal <Variable "current_user"> <String "foo">>
>>> set_condition(condition)

So condition represents an stateless object which the authorization library uses when it needs to find what requests should be allowed or forbidden. Internally, it executes condition by passing all the environment variables, so all the operations inside condition can find if they are met or not, like this:

>>> environment1 = {'user': "gustavo"}
>>> environment2 = {'user': "foo"}
>>> condition(environment1)
>>> condition(environment2)


The general features of Booleano include:

  • Supported operands: Strings, numbers, sets, variables and functions.
  • Supported operations:
    • Relationals (“equals to”, “not equals”, “less than”, “greater than”, “less than or equal to” and “greater than or equal to”).
    • Membership (“belongs to” and “is subset of”).
    • Logical connectives: “not”, “and”, “xor” and “or”.
  • Supports Python 2.4 through 2.6.
  • Comprehensive unit test suite, which covers the 100% of the package.
  • Freedomware, released under the MIT/X License.

While the parser-specific features include:

  • The operands can be bound to identifiers.
  • The identifiers can be available under namespaces.
  • Boolean expressions can contain any Unicode character, even in identifiers.
  • It’s easy to have one grammar per localization.
  • The grammar is adaptive. You can customize it with simple parameters or via Pyparsing.
  • Expressions can span multiple lines. Whitespace makes no difference, so they can contain tabs as well.
  • No nesting level limit. Expressions can be a deep as you want.



Booleano has just gotten its first developers’ preview release and its documentation is far from complete, in spite of the package being absolutely usable. This issue will be solved by the second alpha release.

In the mean time, the best you can do to understand and try Booleano, is to read the rough draft of the tutorial you’re interested in to learn the steps to set up a project and complement this information with the API documentation if necessary. Also, you can always use the mailing list when you need help.