I just finished reading *The Python Corner*’s post “Lambdas and Functions in Python.” The post acts as an introduction to the use of functions as first-class objects in python. The demo code is the implementation of a “Reverse Polish Notation” calculator.

I had never heard of reverse polish notation(RPN) before this post. The short explanation of RPN is available on Wikipedia:

In reverse Polish notation, the operators follow their operands; for instance, to add 3 and 4, one would write 3 4 + rather than 3 + 4.

In that blog post RPN is implemented as a stack of operands and after an operator is pushed onto the stack, the `compute()`

method is called which triggers the evaluation of the `lambda`

specified by the operator. Like this:

The point of the post is to show a python dict using operands as keys with lambdas as values; this demonstrates that lambdas are functions and functions are first-class objects. This allows the `compute`

method of the `RPNEngine`

class to look up a lambda in a dict, and `pop()`

off the stack using the `signature`

function of the `inspect`

module to determine how many arguments are needed for a particular lambda. From there, lambda evaluation is handed off to helper functions named, for instance, `compute_operation_with_two_operands`

and `compute_operation_with_one_operand`

## Currying ¶

One other functional concept that could have helped the example code is that of *currying*. Currying involves changing a function with multiple arity into a series of evaluations of multiple functions each with an arity of 1.

This is a fancy way to say:

```
add = lambda x, y: x + y
add_curried = lambda x: lambda y: x + y
assert(add(2, 2) == add_curried(2)(2))
```

By turning the `compute_operation_with_`

*n*`_operands`

-type functions into curried functions, the code gets much cleaner. That is, instead of a switch like:

```
if number_of_operands == 2:
self.compute_operation_with_two_operands(self.catalog[operation])
if number_of_operands == 1:
self.compute_operation_with_one_operand(self.catalog[operation])
```

You can implement a curried function using a callable python object and do something like:

```
func = self.catalog[operation]
while not func.resolved:
func(self.pop())
```

This gets rid of the clunky `compute_operation_with_`

*n*`_operands`

functions. Here is the full code for a solution using currying:

```
#!/usr/bin/env python3
"""
Engine class for RPN Calculator
"""
import math
from functools import partial
from inspect import signature
class Curry(object):
"""
Curry a callable
Given a callable, returns a an object that can be used like a curried
callable.
>>> c1 = Curry(lambda x, y: x + y)
>>> c2 = Curry(lambda x, y: x + y)
>>> c1(2, 2) == c2(2)(2)
True
:func: callable
"""
def __init__(self, func):
self.func = func
self.argc = len(signature(self.func).parameters)
self.resolved = False
self.answer = None
def __call__(self, *args):
if len(args) == self.argc:
self.answer = self.func(*args)
self.resolved = True
for arg in args:
self.func = partial(self.func, arg)
self.argc = len(signature(self.func).parameters)
return self
class RPNEngine(object):
"""
Reverse Polish Notation (RPN) Engine
A RPN calculator
>>> rpn = RPNEngine()
>>> rpn.push(2)
>>> rpn.push(2)
>>> rpn.compute('+') == 4
True
>>> rpn.compute('AC')
>>> rpn.push(2)
>>> rpn.compute('^2') == 4
True
"""
def __init__(self):
self.stack = []
self.functions = self._get_functions()
def _get_functions(self):
return {
'+': Curry(lambda x, y: x + y),
'-': Curry(lambda x, y: x - y),
'*': Curry(lambda x, y: x * y),
'/': Curry(lambda x, y: x / y),
'^2': Curry(lambda x: x * x),
"SQRT": Curry(lambda x: math.sqrt(x)),
"C": Curry(lambda: self.stack.pop()),
"AC": Curry(lambda: self.stack.clear()),
}
def push(self, item):
self.stack.append(item)
def pop(self):
try:
return self.stack.pop()
except IndexError:
pass
def compute(self, operation):
func = self.functions.get(operation)
if not func:
raise BaseException('%s not a valid function' % operation)
if len(self.stack) < func.argc:
raise BaseException(
'%s requires %d operands, %d given' % (
operation,
func.argc,
len(self.stack)
)
)
if func.argc == 0:
func()
while not func.resolved:
func(self.pop())
return func.answer
```

Reading the final code in the *Python Corner* post made me me really itchy to implement the solution I posted here.