Rewrite Multi‑Line Functions Using Python Lambda – Surprising Tricks

Rewrite Multi‑Line Functions Using Lambda

Python's lambda expressions normally support only single expressions, but they can be crafted to mimic multi‑line functions.

def f(): x = 'string' if x.endswith('g'): x = x[:-1] r = '' for i in xrange(len(x)): if x[i] != 'i': r += x[i] return r # Equivalent lambda expression (lambda: ([x for x in x if not x.endswith('g')], r)[-1])()

Ternary Operator Shortcut

Python's ternary syntax b if a else c can be expressed using logical operators.

(a and [b] or [c])[0]

Remove Duplicates with List Comprehensions

Several approaches to deduplicate a string using list comprehensions, reduce, and loops.

# Simple comprehension ''.join(['' if i and j == x[i-1] else j for i, j in enumerate(x)]) # Using reduce from functools import reduce reduce(lambda a, b: a + b if a.endswith(b) else a + b, x, '')

Generate Fibonacci Sequence via List Comprehensions

Fibonacci numbers can be generated by storing intermediate values in a list.

[l.append(l[-1] + l[-2]) or l for l in [[1, 1]] for x in xrange(10)][0]

List Slicing Tricks

Common list operations: copying, removing/replacing elements, adding to the front or back, and reversing.

# Copy list m = l[:] # Replace element l = [4, 5, 6, 7] # Add to front l = [4, 5, 6] + l[:0] # Add to back l = l + [4] # Reverse list l = l[::-1]

Replace Method Bytecode

Python prevents direct replacement of instance methods, but the underlying bytecode can be swapped.

class A(object): def x(self): print "hello" def y(self): print "world" a = A() a.x.im_func = a.y # Raises TypeError # Bytecode replacement a.x.im_func.func_code = a.y.func_code print a.x() # Outputs "world"

Mutable Default Arguments

Using mutable objects as default arguments is risky, yet useful for caching.

def f(n, c={}): if n in c: return c[n] if n < 2: r = 1 else: r = n * f(n-1, c) c[n] = r return r print f(10) # 3628800 print f.func_defaults # ({1: 1, 2: 2, 3: 6, 4: 24, 5: 120, 6: 720, 7: 5040, 8: 40320, 9: 362880, 10: 3628800},) def fib(n, c={}): if n in c: return c[n] if n < 2: r = 1 else: r = fib(n-2, c) + fib(n-1, c) c[n] = r return r print fib(10) # 89 print fib.func_defaults[0].values() # [1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

These snippets demonstrate powerful, albeit sometimes unsafe, Python tricks that can simplify code or improve performance when used judiciously.