5 posts tagged with "software"

Motivation​

This article reflects on the blog posts, The Prebound Method Pattern and The Sentinel Object Pattern, by Brandon Rhodes. I'll briefly summarize the patterns and discuss my thoughts on them.

The Prebound Method Pattern​

This pattern can be observed when using built-in functions such as random and logging. Instead of needing to create a new instance of the class, we can simply call the function directly. This is possible because a default instance is created within the module, and the instance method is assigned to the module's global namespace.

For example, a logger could be created as follows:

class Logger:
    def __init__(self, name):
        self.name = name

    def log(self, message):
        print(f"{self.name}: {message}")

_default_logger = Logger("default")
log = _default_logger.log

The log method is assigned to the module's global namespace, allowing it to be called directly. This is a simple example, but it's useful when you want to create and use a default instance of a class without needing to create a new one.

This supports the usage of the logger as follows:

import logger

logger.log("Hello World")

This pattern isn't super common, but it's a neat trick to know about. It feels a bit like the singleton pattern. However, since we don't restrict the number of instances that can be created, it's not truly a singleton.

The Sentinel Object Pattern​

This pattern highlights that, despite Python's support for None, we can sometimes provide a more meaningful value to represent a missing value. This is particularly useful when we need to differentiate between a missing value and a valid one.

To illustrate with a similar example from the original article, consider the context of open-source software, where we might want to specify the type of license. We could use None to represent an unspecified license type. However, a valid alternative could be to assign it to a License object that clearly indicates the type of license (e.g., "not specified" or "unlicensed", which may mean different things).

To give a similar example provided in the original article, suppose in the context of open source software, we want to provide a value of the license type. We can use None to represent the case where the license type is not specified. However, a valid alternative could be assigning it to a License object that clearly indicates the type of license (whether it is "not specified" or "unlicensed", they may mean different things).

Here's an example of the pattern in action:

class License:
    def __init__(self, name):
        self.name = name

    def get_name(self):
        return self.name

class Package:
    def __init__(self, name, license):
        self.name = name
        self.license = license

# INSTEAD OF
packages = [
    Package("dummy1", None),
    Package("dummy2", License("BSD")),
]

for package in packages:
    if package.license is None:
        print("not specified")
    else:
        print(package.license.get_name())

# WE CAN USE
UNLICENSED = License("unlicensed")
NOT_SPECIFIED = License("not specified")

packages = [
    Package("dummy1", UNLICENSED),
    Package("dummy2", NOT_SPECIFIED),
    Package("dummy3", License("BSD")),
]

for package in packages:
    print(package.license.get_name())

The advantage here is replacing None with more explicit values, which documents the intent more clearly. This may also reduce the need for None checks in the code.

Conclusion​

The two patterns are subtle but can be quite useful in certain cases. It's good to be aware of them and use them when appropriate. The original articles are also worth reading for more detailed explanations.

Exceptions are a common feature in popular languages like Python and Java. They serve to alter program execution under "exceptional" circumstances. However, handling them is not always straightforward. The concerns revolve around the following:

• When an invoked function throws an exception, how should it be handled?
  • Should you catch and handle it?
  • Or let it bubble up to the caller? Does the caller's caller then need to worry about the exception? (a recursive question)

The problem is exacerbated because you can't simply avoid exceptions. Language libray functions often throw them (consider file handling, conversion, etc.). Moreover, writing exceptions is sometimes necessary, especially when dealing with external user input where "weird" and "unacceptable" cases may arise frequently. Furthermore, once you've written code that throws exceptions, you're likely to invoke that code yourself, necessitating handling your own exceptions. Poorly written code in this regard leaves no one to blame but ourselves.

The article Vexing exceptions by Eric Lippert is an interesting read on this topic. It classifies exceptions into four categories and suggests ways to handle (or not handle) them: fatal, boneheaded, vexing, and exogenous. A quick summary is provided in this post by Stephen Cleary. I'll briefly discuss what I learned from it.

The vexing exception is particularly interesting. Consider these two C# function signatures for parsing a string into an integer:

public static int Parse(string s)

public static bool TryParse(string s, out int result)

Invoking the first function, Parse, usually necessitates exception handling, as it will throw an exception if the input string is not convertible. An alternative approach when dealing with such functions is to seek or implement a variant like TryParse, which doesn't throw exceptions. TryParse returns a success indicator and the operation result. In cases of exceptions, it returns a failure indicator and a default value.

Here's an example in Python:

# Original
def parse_int(s):
    return int(s)

# Usage
try:
    result = parse_int("123")
    print(result)
except ValueError:
    print("Invalid input")

Using the TryParse variant will eliminate the need for exception handling, but it will require an if-else block to manage the success/failure case.

# TryParse
def try_parse_int(s):
    try:
        return True, int(s)
    except ValueError:
        return False, None

# Usage
success, result = try_parse_int("123")
if success:
    print(result)
else:
    print("Invalid input")

In summary, I think of vexing exceptions as "errors that are reasonably likely to occur". If so,

• Use a "Try" variant without exceptions if available.
• Implement a "Try" variant without exceptions if possible.
• If neither is feasible, catch and handle (or re-raise) the exception.

Lastly, "Exogenous" exceptions, the siblings of vexing exceptions, are those thrown by code that you cannot reasonably control. A typical example is file handling functions. It's impractical to ascertain if a file exists before accessing it. Therefore, using code that does file handling likely requires try-catch blocks for possible exceptions. This differs from "Fatal" exceptions, where there's little you can do, while with exogenous exceptions, such as a file not being found, you can handle the situation, perhaps by creating a new file.

Here's a quick flowchart to summarize the ways to handle exceptions: