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Python String Between, Before and After methods

This Python 3 article implements between, before and after methods to find relative substrings. It locates substrings based on surrounding chars.

Between, before, after.

Often in parsing text, a string's position relative to other characters is important. For example, a parameter to a method is between two parenthesis.

For simple text languages, these surrounding characters are easy to handle. We can implement a between() method that returns a substring surrounded by two others. This simplifies logic.

Our methods. Let us start. This Python program includes three new methods: between, before and after. We internally call find() and rfind(). These methods return -1 when nothing is found.

And: We use the indexes returned by find() and rfind to get the indexes of the desired string slice.


Finally: We use the string slice syntax in Python to get substrings of the strings. We thus get substrings relative to other substrings.

Based on:

Python 3

Python program that locates relative slices

def between(value, a, b):
    # Find and validate before-part.
    pos_a = value.find(a)
    if pos_a == -1: return ""
    # Find and validate after part.
    pos_b = value.rfind(b)
    if pos_b == -1: return ""
    # Return middle part.
    adjusted_pos_a = pos_a + len(a)
    if adjusted_pos_a >= pos_b: return ""
    return value[adjusted_pos_a:pos_b]

def before(value, a):
    # Find first part and return slice before it.
    pos_a = value.find(a)
    if pos_a == -1: return ""
    return value[0:pos_a]

def after(value, a):
    # Find and validate first part.
    pos_a = value.rfind(a)
    if pos_a == -1: return ""
    # Returns chars after the found string.
    adjusted_pos_a = pos_a + len(a)
    if adjusted_pos_a >= len(value): return ""
    return value[adjusted_pos_a:]

# Test the methods with this literal.
test = "DEFINE:A=TWO"
print(between(test, "DEFINE:", "="))
print(between(test, ":", "="))

print(before(test, ":"))
print(before(test, "="))

print(after(test, ":"))
print(after(test, "DEFINE:"))
print(after(test, "="))



An issue. The between, before and after methods here return an empty string literal when no value is found. A None result is probably a better choice for these error cases.

So: If the None constant is desired, the return statements could be easily modified to return None instead.


Simple parsers. I have not needed this code for any Python programs. But often in code I have found that implementing small, domain-specific languages is a good strategy.

And: A program could implement a language, like the one with the DEFINE example, to change settings at runtime.

Tip: This could reduce changes to your Python code. If a variable needs changing, you could just alter the text file.

Finally: Reducing code churn, as with a small configuration file language, can reduce the possibility of new bugs being introduced.

Substring logic can be complex—it is a disaster to be off by one character. Your entire program might fail and ruin your day. With methods that handle this logic, things are easier.