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# lisper learning python, ... could you please comment my first python program?

Hi,
to extend my skills, I am learning python. I have written small
program which computes math expression like "1+2*sin(y^10)/cos(x*y)"
and similar, so far only + - * / ^ sin con tan sqrt are supported. But
my program is quite inextensible, I have to change the code to add new
functions... Could some fellow experienced pythonista give me some
tips how to make my program shorter, and more extensible?

to use it, try something like compute("1+x+sin(x)", {"x" : 10}), the
second param is environment so variables like x or y are looked for
value here...

below is the code.... thanks for any tips!

----- code here ----
import sys
import string
import math

def normalize(string):
tmp = "".join([c for c in string if c != " "])
return "(" + tmp + ")"

def most_nested_expression(string):
start = index = 0
end = len(string) - 1
level = max_level = 0
most_nested = False
for c in string:
if c == "(":
level += 1
if level max_level:
most_nested = True
max_level = level
start = index
elif c == ")":
level -= 1
if most_nested == True:
most_nested = False
end = index
index += 1
if level != 0:
raise IOError("mismatched parens")
if max_level == 0:
return (0, len(string), string)
else:
return (start + 1, end - start - 1, string[start + 1:end])

def is_reduced_expression(string):
for c in string:
if c == "^" or c == "*" or c == "/" or c == "+" or c == "-":
return False
return True

def find_first(string, operators):
positions = []
for op in operators:
pos = string.find(op)
if pos != -1:
positions += [pos]
if positions == []:
return None
else:
return min(positions)

def find_operator(string):
for ops in [["^"], ["*", "/"], ["+", "-"]]:
pos = find_first(string, ops)
if pos != None:
if string[pos + 1] == "+" or string[pos + 1] == "-":
return pos + 1
else:
return pos
return None

def left_operand(string, operator_pos):
left = None
operator = string[operator_pos]
candidates = [pos for pos in [string.rfind(op, 0, operator_pos)
for op in ["(", ")", "^", "*", "/",
"+", "-"]]
if pos != -1]
if candidates != []:
left = max(candidates)
if left == None:
if operator == "^" or operator == "*" or operator == "/":
raise IOError("invalid expression %s" % string)
else: # + or -
return ("0", operator_pos)
else:
if left + 1 == operator_pos:
if operator == "+" or operator == "-":
return ("0", left)
else:
raise IOError("invalid expression %s" % string)
else:
return (string[left + 1:operator_pos], left)

def right_operand(string, operator_pos):
right = None
candidates = [pos for pos in [string.find(op, operator_pos + 1)
for op in ["(", ")", "^", "*", "/",
"+", "-"]]
if pos != -1]
if candidates == []:
if operator_pos == len(string) - 1:
raise IOError("invalid expression %s" % string)
else:
return (string[operator_pos + 1:], len(string))
else:
right = min(candidates)
if operator_pos + 1 == right:
raise IOError("invalid expression %s" % string)
else:
return (string[operator_pos + 1:right], right)

def function_name(string, left_paren_pos):
candidates = [pos for pos in [string.rfind(op, 0, left_paren_pos)
for op in ["(", "^", "*", "/", "+",
"-"]]
if pos != -1]
if candidates == []:
return (None, None)
else:
left = max(candidates)
name = string[left + 1:left_paren_pos]
fun_names = ["sin", "cos", "tan", "sqrt"]

for f in fun_names:
if f == name:
return (left + 1, name)
return (None, None)

def reduce_step(string, index):
(left, exp_len, exp) = most_nested_expression(string)
#print "most nested %s" % exp
if is_reduced_expression(exp):
(left1, name) = function_name(string, left - 1)
if left1 != None:
return ((name, string[left:left + exp_len], None),
string[0:left1] + "\$%s" % index + string[left +
exp_len + 1:],
True)
else:
return ((None, None, None), string[0:left - 1] + exp +
string[left + exp_len + 1:], False)
else:
operator_pos = find_operator(exp) + left
(left_op, left_mark) = left_operand(string, operator_pos)
(right_op, right_mark) = right_operand(string, operator_pos)
return ((string[operator_pos], left_op, right_op),
string[0:left_mark + 1] + "\$%s" % index +
string[right_mark:],
True)

def reduce(string):
chain = []
index = 0
while string[0] == "(":
((function, left_op, right_op), new_string, is_expr) =
reduce_step(string, index)
if is_expr:
chain += [(function, left_op, right_op)]
index += 1
string = new_string
return chain

def add(a, b): return a + b
def sub(a, b): return a - b
def mul(a, b): return a * b
def div(a, b): return a / b

def translate_function(fn_str):
if fn_str == "+": return add
elif fn_str == "-": return sub
elif fn_str == "*": return mul
elif fn_str == "/": return div
elif fn_str == "^": return math.pow
elif fn_str == "sin": return math.sin
elif fn_str == "cos": return math.cos
elif fn_str == "tan": return math.tan
elif fn_str == "sqrt": return math.sqrt
else: raise IOError("unknown function %s" % fn_str)

def translate_operand(op_str):
if op_str[0] == "\$":
result_idx = int(op_str[1:])
return lambda results, env: results[result_idx]
else:
try:
value = float(op_str)
return lambda results, env: value
except ValueError:
return lambda results, env: env[op_str]

def translate(chain):
res = []
for (fn_str, left_op_str, right_op_str) in chain:
fn = translate_function(fn_str)
left_op = translate_operand(left_op_str)
if right_op_str != None:
res += [(fn, [left_op, translate_operand(right_op_str)])]
else:
res += [(fn, [left_op])]
return res

def compute_value(chain, env, results):
assert len(chain) == len(results)
index = 0
for (fn, operand_fns) in chain:
operands = [op_fn(results, env) for op_fn in operand_fns]
results[index] = apply(fn, operands)
index += 1
return results[index - 1]

def compute(string, env):
print "input: %s" % string
string = normalize(string)
print "normalized: %s" % string
string_chain = reduce(string)
print "reduced: %s" % string_chain
chain = translate(string_chain)
print "translated: %s" % chain
return compute_value(chain, env, range(len(chain)))

Aug 26 '07 #1
4 1378
On Sun, 26 Aug 2007 15:56:08 +0000, neptundancer wrote:
Hi,
to extend my skills, I am learning python. I have written small
program which computes math expression like "1+2*sin(y^10)/cos(x*y)" and
similar, so far only + - * / ^ sin con tan sqrt are supported. But my
program is quite inextensible, I have to change the code to add new
functions... Could some fellow experienced pythonista give me some tips
how to make my program shorter, and more extensible?
Just a few comments at random. This is certainly not meant to be
exhaustive:

def normalize(string):
tmp = "".join([c for c in string if c != " "])
return "(" + tmp + ")"

def normalize(astring):
return "(" + astring.replace(" ", "") + ")"
def most_nested_expression(string):
[snip code]
if level != 0:
raise IOError("mismatched parens")
You raise IOError quite often, but that's a misuse of it. IOError is a
subclass of EnvironmentError, and is meant to indicate (e.g.) a failed

If you execute help(IOError) at the interactive prompt, you will see:

class IOError(EnvironmentError)
| I/O operation failed.
I suggest a better exception to use would be ValueError, or even create

class MyCustomError(ValueError):
pass

would be a minimal example.

def is_reduced_expression(string):
for c in string:
if c == "^" or c == "*" or c == "/" or c == "+" or c == "-":
return False
return True
Change that to:

for c in string:
if c in "^*/+-": return False
return True
[snip]

def add(a, b): return a + b
def sub(a, b): return a - b
def mul(a, b): return a * b
def div(a, b): return a / b
Replace the above four functions with:

from operator import add, sub, mul
from operator import truediv as div
def translate_function(fn_str):
if fn_str == "+": return add
elif fn_str == "-": return sub
elif fn_str == "*": return mul
elif fn_str == "/": return div
elif fn_str == "^": return math.pow
elif fn_str == "sin": return math.sin elif fn_str == "cos": return
math.cos elif fn_str == "tan": return math.tan elif fn_str ==
"sqrt": return math.sqrt else: raise IOError("unknown function %s" %
fn_str)

fn_map = {"+": add, "-": sub, "*": mul, "/": div,
"^": math.pow, "sin": math.sin } # etc.

def translate_function(fn_str):
try:
return fn_map[fn_str]
except KeyError:
raise ValueError("unknown function '%s'" % fn_str)
Hope that helps.
--
Steven.
Aug 26 '07 #2
Thanks a lot for comments! I am going to fix the source according to

Nep

On Aug 26, 6:32 pm, Steven D'Aprano <st...@REMOVE-
THIScybersource.com.auwrote:
On Sun, 26 Aug 2007 15:56:08 +0000, neptundancer wrote:
Hi,
to extend my skills, I am learning python. I have written small
program which computes math expression like "1+2*sin(y^10)/cos(x*y)" and
similar, so far only + - * / ^ sin con tan sqrt are supported. But my
program is quite inextensible, I have to change the code to add new
functions... Could some fellow experienced pythonista give me some tips
how to make my program shorter, and more extensible?

Just a few comments at random. This is certainly not meant to be
exhaustive:
def normalize(string):
tmp = "".join([c for c in string if c != " "])
return "(" + tmp + ")"

def normalize(astring):
return "(" + astring.replace(" ", "") + ")"
def most_nested_expression(string):
[snip code]
if level != 0:
raise IOError("mismatched parens")

You raise IOError quite often, but that's a misuse of it. IOError is a
subclass of EnvironmentError, and is meant to indicate (e.g.) a failed

If you execute help(IOError) at the interactive prompt, you will see:

class IOError(EnvironmentError)
| I/O operation failed.

I suggest a better exception to use would be ValueError, or even create

class MyCustomError(ValueError):
pass

would be a minimal example.
def is_reduced_expression(string):
for c in string:
if c == "^" or c == "*" or c == "/" or c == "+" or c == "-":
return False
return True

Change that to:

for c in string:
if c in "^*/+-": return False
return True

[snip]
def add(a, b): return a + b
def sub(a, b): return a - b
def mul(a, b): return a * b
def div(a, b): return a / b

Replace the above four functions with:

from operator import add, sub, mul
from operator import truediv as div
def translate_function(fn_str):
if fn_str == "+": return add
elif fn_str == "-": return sub
elif fn_str == "*": return mul
elif fn_str == "/": return div
elif fn_str == "^": return math.pow
elif fn_str == "sin": return math.sin elif fn_str == "cos": return
math.cos elif fn_str == "tan": return math.tan elif fn_str ==
"sqrt": return math.sqrt else: raise IOError("unknown function %s" %
fn_str)

fn_map = {"+": add, "-": sub, "*": mul, "/": div,
"^": math.pow, "sin": math.sin } # etc.

def translate_function(fn_str):
try:
return fn_map[fn_str]
except KeyError:
raise ValueError("unknown function '%s'" % fn_str)

Hope that helps.

--
Steven.

Aug 26 '07 #3
On Aug 26, 5:56 pm, neptundan...@gmail.com wrote:
Hi,
to extend my skills, I am learning python. I have written small
program which computes math expression like "1+2*sin(y^10)/cos(x*y)"
and similar, so far only + - * / ^ sin con tan sqrt are supported. But
my program is quite inextensible, I have to change the code to add new
functions... Could some fellow experienced pythonista give me some
tips how to make my program shorter, and more extensible?
I understand that you are doing this as a learning exercise. Still, I
would
recommend you to have a look at the shlex module in the standard
library
and to pyparsing. Looking at their source code should should give you
some
idea. BTW, since you are familiar with Lisp, I would recommend you
IPython
for a better interactive experience.

Michele Simionato

Aug 26 '07 #4
On Aug 26, 7:40 pm, Michele Simionato <michele.simion...@gmail.com>
wrote:
On Aug 26, 5:56 pm, neptundan...@gmail.com wrote:
Hi,
to extend my skills, I am learning python. I have written small
program which computes math expression like "1+2*sin(y^10)/cos(x*y)"
and similar, so far only + - * / ^ sin con tan sqrt are supported. But
my program is quite inextensible, I have to change the code to add new
functions... Could some fellow experienced pythonista give me some
tips how to make my program shorter, and more extensible?

I understand that you are doing this as a learning exercise. Still, I
would
recommend you to have a look at the shlex module in the standard
library
and to pyparsing. Looking at their source code should should give you
some
idea. BTW, since you are familiar with Lisp, I would recommend you
IPython
for a better interactive experience.

Michele Simionato
thanks for the tip! Now I have IPython working nicely. The shell now
looks a little like Christmas tree but I think it can be reduced a
bit ;)
the ?var and auto completion is exactly what I was looking for!

Nep

Aug 26 '07 #5

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