From ee26c4658cb10ef4f917080cc84bef1caf0a8d14 Mon Sep 17 00:00:00 2001
From: Christopher Bohn <bohn@unl.edu>
Date: Wed, 17 Mar 2021 13:42:38 -0500
Subject: [PATCH] Introduced additional error handling and doctests to parse
function
---
MutualRecursion.py | 51 ++++++++++++++++++++++++++++++++++++++++++----
1 file changed, 47 insertions(+), 4 deletions(-)
diff --git a/MutualRecursion.py b/MutualRecursion.py
index 016a1cc..4e1d2c5 100755
--- a/MutualRecursion.py
+++ b/MutualRecursion.py
@@ -7,10 +7,45 @@ def parse(expression):
"""
Expects an expression in the form of left-paren left-expression operator right-expression right-paren,
where each left/right expression is either an integer or another expression.
- For example, '(1+2)' or '((1+2)-(3*4))'
+
+ Expressions may have literal sub-expressions
+ >>> parse('(1 + 2)')
+ ('1', '+', '2')
+
+ And an expression may have non-literal sub-expressions
+ >>> parse('((1+2) - (3*4))')
+ ('(1+2)', '-', '(3*4)')
+
+ Spaces may be included in the expression
+ >>> parse('(1 - (3 * 4))')
+ ('1', '-', '(3*4)')
+
+ But spaces are not required
+ >>> parse('((1+2)-3)')
+ ('(1+2)', '-', '3')
+
+ The expression must be enclosed within parentheses
+ >>> parse('1+2')
+ Traceback (most recent call last):
+ ...
+ SyntaxError: Expression must be bounded by parenthesis. 1+2 does not satisfy.
+
+ And the parentheses must be balanced
+ >>> parse('((1+2)')
+ Traceback (most recent call last):
+ ...
+ SyntaxError: Expression must have balanced parentheses. ((1+2) does not satisfy.
+
+ Finally, the expression must have two sub-expressions separated by one an arithmetic operator from {+ - * /}
+ >>> parse('(3 3)')
+ Traceback (most recent call last):
+ ...
+ SyntaxError: Expression must have an operator (3 3) does not satisfy.
"""
+ if expression == '':
+ raise SyntaxError('Vacuous expressions are disallowed.')
if expression[0] != '(' or expression[-1] != ')':
- raise SyntaxError(f'Expression must be bounded by parenthesis. ${expression} does not satisfy.')
+ raise SyntaxError(f'Expression must be bounded by parenthesis. {expression} does not satisfy.')
full_expression = expression.replace(' ', '')[1:-1]
if full_expression[0] != '(': # the left expression is an integer
index = math.inf
@@ -18,6 +53,8 @@ def parse(expression):
this_operator_index = full_expression.find(operator)
if -1 < this_operator_index < index:
index = this_operator_index
+ if index == math.inf:
+ raise SyntaxError(f'Expression must have an operator {expression} does not satisfy.')
else:
parentheses_count = 1
index = 1
@@ -27,9 +64,15 @@ def parse(expression):
if full_expression[index] == ')':
parentheses_count -= 1
index += 1
+ if index == len(full_expression) and parentheses_count > 0:
+ raise SyntaxError(f'Expression must have balanced parentheses. {expression} does not satisfy.')
left_expression = full_expression[:index]
- operator = full_expression[index]
- right_expression = full_expression[index + 1:]
+ try:
+ operator = full_expression[index]
+ right_expression = full_expression[index + 1:]
+ except IndexError:
+ raise SyntaxError(f'Non-literal expression must have two subexpressions and an operator. '
+ f'{expression} does not satisfy.')
return left_expression, operator, right_expression
--
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