Use Lambda Inline

use-lambda-inline

When to Use

• Simple one-liner needed inline
• key= argument to sorted/max/min
• Dict of operation functions
• Trivial transformations
• Callback registration

When NOT to Use

• Multiple statements needed
• Function will be reused by name
• Complex logic that needs documentation
• Recursion needed (lambda can't easily recurse)

The Pattern

Use lambda for anonymous, single-expression functions.

# Sorting with custom key
sorted(items, key=lambda x: x.priority)
max(scores, key=lambda s: s.value)

# Dict of operations
ops = {'+': lambda a, b: a + b,
       '-': lambda a, b: a - b,
       '*': lambda a, b: a * b}

result = ops['+'](3, 4)  # 7

# Filter with inline predicate
positive = filter(lambda x: x > 0, numbers)

# Default argument for capturing
buttons = [Button(lambda i=i: click(i)) for i in range(10)]

Example (from pytudes)

# Lisp interpreter operations (lis.py)
env = {
    '+': op.add,
    '-': op.sub,
    'apply': lambda proc, args: proc(*args),
    'begin': lambda *x: x[-1],
    'car': lambda x: x[0],
    'cdr': lambda x: x[1:],
    'cons': lambda x, y: [x] + y,
    'length': len,
    'list': lambda *x: list(x),
    'map': lambda *args: list(map(*args)),
    'null?': lambda x: x == [],
    'number?': lambda x: isinstance(x, Number),
}

# Probability key functions (Probability.ipynb)
max(candidates, key=lambda c: probability(c))

# MRV heuristic (Sudoku.ipynb)
s = min((s for s in squares if len(values[s]) > 1),
        key=lambda s: len(values[s]))

# Sort by multiple criteria
sorted(items, key=lambda x: (x.category, -x.priority, x.name))

# Regex substitution callback (Cryptarithmetic.ipynb)
result = re.sub(r'[A-Z]+',
                lambda m: translate_word(m.group()),
                formula)

Key Principles

1. One expression only: No statements, just return value
2. Use op module for operators: op.add clearer than lambda a,b: a+b
3. Default args for capture: lambda i=i: captures current i
4. Keep it short: If complex, use named function
5. Readable keys: key=lambda x: x.name is clear