Python One Line if elif else

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πŸ’‘ Problem Formulation: A common Python scenario to assign a value to a variable based on a condition. The basic if-elif-else construct is bulky and may not always be the most efficient way to handle simple conditional assignments.

Consider a situation where you have a numerical input, and you want to categorize it as ‘small’, ‘medium’, or ‘large’ based on its value. Using a traditional multi-line if-elif-else block can be clunky when a one-liner can offer a more concise solution.

Method 1: Conditional Expressions (Ternary Operator)

A conditional expression, also known as the ternary operator in Python, allows a simple if-else condition to be condensed into a single line. The syntax is value_if_true if condition else value_if_false. However, this method does not directly extend to elif but can be nested for similar functionality.

Here’s an example:

x = int(input())
size = 'small' if x < 10 else 'medium' if x < 20 else 'large'

In this code snippet, we assign the string ‘small’ to the variable size if x is less than 10, ‘medium’ if x is between 10 and 20, and ‘large’ if x is 20 or greater. This combines conditionals in a single, readable line.

πŸ‘‰ Python Nested Multiple Ternary Operators

Method 2: Dictionary Mapping

Dictionary mapping can emulate switch-case behavior found in other languages. It’s a method of linking keys to values and can be used to associate conditions with corresponding outcomes.

Here’s an example:

size = {True: 'small', False: {True: 'medium', False: 'large'}}[(x < 10) or (10 <= x < 20)]

This snippet uses a nested dictionary where the keys are the results of the condition checks. The outer dictionary splits the size between ‘small’ and not ‘small’, and the inner dictionary further refines the result to ‘medium’ or ‘large’.

Method 3: Using Lambda Functions

Lambda functions, or anonymous functions, are small, one-time use functions defined on the fly. They can be used within a conditional expression to mimic the elif functionality on one line.

Here’s an example:

size = (lambda: 'large', lambda: 'medium', lambda: 'small')[2 - (x >= 10) - (x >= 20)]()

Here, we create a tuple of lambda functions that return the respective size strings. We calculate the index to access the right lambda based on the value of x, and then immediately call the selected lambda function.

Method 4: List Comprehension With Conditions

While list comprehensions are typically used for creating lists, they can be combined with conditions to simulate a one-line if-elif-else. This method abuses the fact that list comprehensions can have conditional outputs.

Here’s an example:

size = [outcome for condition, outcome in [(x < 10, 'small'), (x < 20, 'medium')] if condition] or ['large']

In this pattern, we construct a list comprehension that evaluates each condition in sequence and outputs the corresponding outcome. If none match, the resulting list is empty, and we fall back to ‘large’ due to the logic provided by or.

Bonus One-Liner Method 5: Using next() and Generators

Generators express an on-the-fly iteration mechanism, and when combined with the next() function, it can be used to implement the first match in if-elif-else logic.

Here’s an example:

size = next((outcome for condition, outcome in [(x < 10, 'small'), (x >= 10 and x < 20, 'medium'), (x >= 20, 'large')] if condition), 'unknown')

In this one-liner, a generator expression checks each condition sequence and yields the outcome for the first True condition. If no conditions are met, next() falls back to ‘unknown’.

πŸ‘‰ Understanding Generators In Python


  • Method 1: Conditional Expressions
    • Strength: Extremely concise for simple conditions.
    • Weakness: Readability decreases with complexity; nesting can be confusing.
  • Method 2: Dictionary Mapping
    • Strength: Emulates switch-case; can be clean if used properly.
    • Weakness: Overly complex for small sets of conditions.
  • Method 3: Using Lambda Functions
    • Strength: Offers compact and functional-style syntax.
    • Weakness: Can seem cryptic and may sacrifice readability.
  • Method 4: List Comprehension With Conditions
    • Strength: Hack of list comprehensions; novel approach.
    • Weakness: Abuse of list comprehension purpose; not straightforward.
  • Bonus One-Liner Method 5: Using next and Generators
    • Strength: Cleanly matches first true condition; defaults possible with next().
    • Weakness: Still a bit arcane for those unfamiliar with generator expressions.

In some scenarios, the traditional multiline if-elif-else may still be the most readable choice, especially for complex conditions or when working with teams who may not be familiar with more advanced Python constructs.

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