Using strings

Since Python strings are immutable, they're an excellent example of functional programming objects. A Python str object has a number of methods, all of which produce a new string as the result. These methods are pure functions with no side effects.

The syntax for str method functions is postfix, where most functions are prefix. This means that complex string operations can be hard to read when they're co-mingled with conventional functions. For example, in this expression, len(variable.title()), the title() method is in postfix notation and the len() function is in prefix notation.

When scraping data from a web page, we may have a function to clean the data. This could apply a number of transformations to a string to clean up the punctuation and return a Decimal object for use by the rest of the application. This will involve a mixture of prefix and postfix syntax.

It could look like the following command snippet:

from decimal import *
from typing import Text, Optional
def clean_decimal(text: Text) -> Optional[Text]:
    if text is None: return None
    return Decimal(
        text.replace("$", "").replace(",", "")) 

This function does two replacements on the string to remove $ and , string values. The resulting string is used as an argument to the Decimal class constructor, which returns the desired object. If the input value is None, this is preserved; this is why the Optional type hint is used.

To make the syntax look more consistent, we can consider defining our own prefix functions for the string method functions, as follows:

def replace(str: Text, a: Text, b: Text) -> Text:
    return str.replace(a,b)

This can allow us to use Decimal(replace(replace(text, "$", ""), ",", "")) with consistent-looking prefix syntax. It's not clear whether this kind of consistency is a significant improvement over the mixed prefix and postfix notation. This may be an example of a foolish consistency.

A slightly better approach may be to define a more meaningful prefix function to strip punctuation, such as the following command snippet:

def remove(str: Text, chars: Text) -> Text:
    if chars: 
        return remove(
            str.replace(chars[0], ""), 
            chars[1:]
        )
    return str

This function will recursively remove each of the characters from the chars variable. We can use it as Decimal(remove(text, "$,")) to make the intent of our string cleanup more clear.