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Tips and Tricks

The following section lists a series of tips and tricks I found useful in my experience.

This includes:

  • Solutions to language problems
  • Tips for using the wsadmin

NameError with Boolean values

Being Python 2.7 the earliest Python version i was used to work on, i was shocked to find that simple scripts running on Python 2.1 failed with NameError: True and NameError: False.

Upon investigation I found that:

  • > 3.0: True and False are reserved keywords (and so cannot be defined) and in Python;
  • >= 2.3: True and False are defined simply as constants, so they could even be assigned arbitrary values;
  • < 2.3: True and False are NOT defined at all, hence the NameError exception.

Info

More informations on this subject and other interesting considerations can be found in this article.

Solutions

Since boolean values were introduced in Python 2.3 (with PEP 285), if the wsadmin is running an older Jython version, you won't be able to use True or False (nor the bool(...) function) in your scripts.

To overcome this problem you have 2 options:

  1. Provide a polyfill for True and False
  2. Use 0 (falsy) and 1 (truthy)

Let's explore these options in detail...

1. Define True/False

To use True and False in the scripts add the following line at the top of your code, then you'll be able to write your scripts as usual: 

exec("try: (True, False)\nexcept NameError: exec('True = 1==1; False = 1==0')")

Info

The code is wrapped in an exec(...) function for two primary reasons:

  1. To provide a oneliner fix, easier to copy-paste, thus reducing errors;
  2. To elude linters and static type checkers like pylint or mypy which consider this an error (while it is only a false positive, since we're dealing with old legacy Python versions).
  3. The inner exec call serves the sole purpose of safe guarding in case the script is being executed (while testing) on Python 3.x (the SyntaxError raised when trying to assign a keyword cannot be catched).

Source

This workaround is inspired by this SO answer.

Example

The following example shows how the False value gets correctly evaluated in a condition: 

>>> exec("try: (True, False)\nexcept NameError: exec('True = 1==1; False = 1==0')")
>>> exists = True

>>> if exists:
...     print("File exists.")
... else:
...     print("File does not exists.")
File exists.

2. Use 0/1

Python interprets 0 and 1 respectively as falsy and truthy values. This means that we can use 0 instead of False and 1 instead of True.

Example

We can rewrite the previous example like this: 

>>> exists = 1

>>> if exists:
...     print("File exists.")
... else:
...     print("File does not exists.")
File exists.

The __future__ module

Just like Python, Jython also provides the __future__ module.

This built-in module was added in Python 2.1 and allows the developer to force the Python (and so, Jython) interpreters into using newer features of the language.

Warning

The list of features the module exposes depends on the version of Jython your DMGR is running, so always make sure that the feature you're trying to import is actually present in the __future__ module.

A full list of features available, with all their descriptions and version prerequisites can be found here.

  1. To discover the features available in your Jython environment use the following commands: 

    >>> import __future__
>>> print([feature for feature in dir(__future__) if not feature.startswith("_")])
['nested_scopes']

  2. Then, if you intend to use them, just import them like a regular module: 

    >>> from __future__ import nested_scopes

    Info

    In this case I'm using the nested_scopes feature, which backports PEP 227 (introduced in Python 2.2) down to Python 2.1.