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Simple python closure objects
"""
Simple message-passing closure objects example similar to SICP 3.1.1
"""
def make_account(balance):
# In Python 2.x we have to close over a mutable variable, so we are using
# obj.__setitem__(...) assignment rather than regular assignment which
# attempts to create a new variable within the current scope when each
# closure is initially compiled. Otherwise cannot assign the balance value
# with our deposit and withdraw closures.
# In Python 3.x we can use the `nonlocal` keyword (with usage similar to
# `global`) within our closures instead.
current_balance = [balance]
def deposit(amount):
current_balance[0] = current_balance[0] + amount
return current_balance[0]
def withdraw(amount):
if current_balance[0] >= amount:
current_balance[0] = current_balance[0] - amount
return current_balance[0]
return 'Insufficient funds'
def dispatch(method_name):
if method_name == 'deposit':
return deposit
elif method_name == 'withdraw':
return withdraw
elif method_name == 'balance':
return current_balance[0]
else:
raise ValueError('Unknown request -- {}'.format(method_name))
return dispatch
account = make_account(100)
account("deposit")(25)
account("withdraw")(50)
print account("balance")
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