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POPython (sympy) TypeError: cannot concatenate 'str' and 'Add' objects
 

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  1. POPython (sympy) TypeError: cannot concatenate 'str' and 'Add' objects
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    <p>I'm trying to use an equation that I've generated using <code>sympy.Eq(func, var)</code> in a lambda. It seems to be returning a list of 'Add' objects, which I'm not sure how to use. I tried typecasting to a<code>str</code> before using <code>eval</code>, and i tried <code>from sympy import I</code>, which i think has something to do with the problem.</p> <p>Here's the code, and the error message:</p> <blockquote> <p>py solid.py -v y = (x-1.0)<em>(x-3.0)</em>*2 1.0 3.0 -axis y --method disk</p> </blockquote> <hr> <p><strong>Here is a running example:</strong></p> <pre><code>import sys import argparse import math from scipy import integrate from sympy import Symbol, Eq, solve, I import numpy import pylab parser = argparse.ArgumentParser(description='Find the volume of the solid of rotation defined. Use * for multiplication and ** to raise a power. Bounds are set equal to the implicit variable. Trailing .0\'s must be used for constants, and parentheses must be escaped ( e.g. \(x-2\) ). Answers are accurate to 6 decimals') parser.add_argument('var', choices='xy', help='x|y') parser.add_argument('=', metavar='=', choices='=') parser.add_argument('equation', help='continous function') parser.add_argument('a', type=float, help='bound \'a\'') parser.add_argument('b', type=float, help='bound \'b\'') parser.add_argument('-axis', choices='xy', help='axis of revolution (default == x)') parser.add_argument('-m', choices='ds', help='method (disk/shell)') parser.add_argument('-g', action='store_true', help='show graph') parser.add_argument('-v', action='store_true', help='verbose (for debugging)') args = parser.parse_args() y = Symbol('y') x = Symbol('x') def solve_for_implicit(func, var): if var == 'x': equation = Eq(eval(func), x) func = solve(equation, y) else: equation = Eq(eval(func), y) func = solve(equation, x) return func def volume(var, func, a, b, axis=None, method=None): if axis == None: axis = 'x' if method == 's': method = 'shell' if method == 'd': method = 'disk' if var == axis and axis == 'x': if args.v: print 'x = y about x' if not method == 'disk': pass else: pass elif var == axis and axis == 'y': # SHELL METHOD if args.v: print 'y = x about y' if not method == 'disk': # this is what should be used for y = x about y if args.v: print 'using SHELL method' func = eval('lambda x: ' + 'x*(' + func + ')') integral = integrate.quad(func, a, b) if args.v: print 'integral from', a, 'to', b, '=', integral[0] answer = 2 * math.pi * integral[0] if args.v: print '2*pi*', integral[0], '=', answer else: func = solve_for_implicit(func, var) print func[0] func = eval('lambda y: ' + '(' + str(func[0]) + ')**2') integral = integrate.quad(func, a, b) if args.v: print 'integral=', integral[0] answer = math.pi * integral[0] elif not var == axis and axis == 'y': # DISK METHOD if args.v: print 'x = y about y -- ' if not method == 'shell': pass else: pass elif not var == axis and axis == 'x': # DISK if args.v: print 'y = x about x --', if not method == 'shell': pass else: pass return answer print volume(args.var, args.equation, args.a, args.b, args.axis, args.m) if args.g: graph(args.equation, args.a, args.b) </code></pre> <blockquote> <p>Traceback (most recent call last):<br> File "solid.py", line 136, in print volume(args.var, args.equation, args.a, args.b, args.axis, args.m) File "solid.py", line 75, in volume func = eval('lambda y: ' + '(' + func[0] + ')**2') TypeError: cannot concatenate 'str' and 'Add' objects</p> </blockquote> <p>and when I try typecasting it to a str() first i get this:</p> <blockquote> <p>File "solid.py", line 136, in print volume(args.var, args.equation, args.a, args.b, args.axis, args.m) File "solid.py", line 76, in volume integral = integrate.quad(func, a, b) File "/usr/lib/python2.6/dist-packages/scipy/integrate/quadpack.py", line 185, in quad retval = _quad(func,a,b,args,full_output,epsabs,epsrel,limit,points) File "/usr/lib/python2.6/dist-packages/scipy/integrate/quadpack.py", line 249, in _quad return _quadpack._qagse(func,a,b,args,full_output,epsabs,epsrel,limit) quadpack.error: Supplied function does not return a valid float.</p> </blockquote> <p>also, <code>print func[0]</code> gives:</p> <blockquote> <p>2.33333333333333 + (0.296296296296298 - y/2 + (-0.0877914951989024 + (0.592592592592595 - y)<strong>2/4)</strong>(1/2))<strong>(1/3)*(1/2 - I*3</strong>(1/2)/2) + 0.444444444444444/((1/2 - I*3*<em>(1/2)/2)</em>(0.296296296296298 - y/2 + (-0.0877914951989024 + (0.592592592592595 - y)<strong>2/4)</strong>(1/2))**(1/3))</p> </blockquote> <p>thanks for your help everyone, sorry for not providing a complete script before.</p>
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    1. COwhat is type of `func`?
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    2. COfunc is a lambda, like `lambda x: x**2`
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      1. USNona Urbiz
    3. COThat all doesn't fit together. func can't be a function. And the 1st error msg shouldn't come when you get the 2nd with only str(...). Please post a minimal runnable example that reproduces the error. Why are you using eval btw.? Seems to be totally unnecessary for me, makes everything just absolutely unreadable. `func = lambda y: float(func[0])**2` should do the job.
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