Development Tips: Comparisons in Python

Introduction

When debugging comparisons and hashes in SymPy, it is necessary to understand when exactly Python calls each method. Unfortunately, the official Python documentation for this is not very detailed (see the docs for rich comparison, __cmp__() and __hash__() methods).

We wrote this guide to fill in the missing gaps. After reading it, you should be able to understand which methods do (and do not) get called and the order in which they are called.

Hashing

Every Python class has a __hash__() method, the default implementation of which is:

def __hash__(self):
    return id(self)

You can reimplement it to return a different integer that you compute on your own. hash(x) just calls x.__hash__(). Python builtin classes usually redefine the __hash__() method. For example, an int has something like this:

def __hash__(self):
    return int(self)

and a list does something like this:

def __hash__(self):
    raise TypeError("list objects are unhashable")

The general idea about hashes is that if two objects have a different hash, they are not equal, but if they have the same hash, they might be equal. (This is usually called a “hash collision” and you need to use the methods described in the next section to determine if the objects really are equal).

The only requirement from the Python side is that the hash value mustn’t change after it is returned by the __hash__() method.

Please be aware that hashing is platform-dependent. This means that you can get different hashes for the same SymPy object on different platforms. This affects for instance sorting of sympy expressions. You can also get SymPy objects printed in different order.

When developing, you have to be careful about this, especially when writing tests. It is possible that your test runs on a 32-bit platform, but not on 64-bit. An example:

>> from sympy import *
>> x = Symbol('x')
>> r = rootfinding.roots_quartic(Poly(x**4 - 6*x**3 + 17*x**2 - 26*x + 20, x))
>> [i.evalf(2) for i in r]
[1.0 + 1.7*I, 2.0 - 1.0*I, 2.0 + I, 1.0 - 1.7*I]

If you get this order of solutions, you are probably running 32-bit system. On a 64-bit system you would get the following:

>> [i.evalf(2) for i in r]
[1.0 - 1.7*I, 1.0 + 1.7*I, 2.0 + I, 2.0 - 1.0*I

When you now write a test like this:

r = [i.evalf(2) for i in r]
assert r == [1.0 + 1.7*I, 2.0 - 1.0*I, 2.0 + I, 1.0 - 1.7*I]

it will fail on a 64-bit platforms, even if it works for your 32-bit system. You can avoid this by using the sorted() or set() Python built-in:

r = [i.evalf(2) for i in r]
assert set(r) == set([1.0 + 1.7*I, 2.0 - 1.0*I, 2.0 + I, 1.0 - 1.7*I])

This approach does not work for doctests since they always compare strings that would be printed after a prompt. In that case you could make your test print results using a combination of str() and sorted():

>> sorted([str(i.evalf(2)) for i in r])
['1.0 + 1.7*I', '1.0 - 1.7*I', '2.0 + I', '2.0 - 1.0*I']

or, if you don’t want to show the values as strings, then sympify the results or the sorted list:

>> [S(s) for s in sorted([str(i.evalf(2)) for i in r])]
[1.0 + 1.7*I, 1.0 - 1.7*I, 2.0 + I, 2.0 - I]

The printing of SymPy expressions might be also affected, so be careful with doctests. If you get the following on a 32-bit system:

>> print dsolve(f(x).diff(x, 2) + 2*f(x).diff(x) - f(x), f(x))
f(x) == C1*exp(-x + x*sqrt(2)) + C2*exp(-x - x*sqrt(2))

you might get the following on a 64-bit platform:

>> print dsolve(f(x).diff(x, 2) + 2*f(x).diff(x) - f(x), f(x))
f(x) == C1*exp(-x - x*sqrt(2)) + C2*exp(-x + x*sqrt(2))

Method Resolution

Let a, b and c be instances of any one of the Python classes. As can be easily checked by the python script at the end of this guide, if you write:

a == b

Python calls the following – in this order:

a.__eq__(b)
b.__eq__(a)
a.__cmp__(b)
b.__cmp__(a)
id(a) == id(b)

If a particular method is not implemented (or a method returns NotImplemented [1]) Python skips it and tries the next one until it succeeds (i.e. until the method returns something meaningful). The last line is a catch-all method that always succeeds.

If you write:

a != b

Python tries to call:

a.__ne__(b)
b.__ne__(a)
a.__cmp__(b)
b.__cmp__(a)
id(a) == id(b)

If you write:

a < b

Python tries to call:

a.__lt__(b)
b.__gt__(a)
a.__cmp__(b)
b.__cmp__(a)
id(a) < id(b)

If you write:

a <= b

Python tries to call:

a.__le__(b)
b.__ge__(a)
a.__cmp__(b)
b.__cmp__(a)
id(a) <= id(b)

And similarly for a > b and a >= b.

If you write:

sorted([a, b, c])

Python calls the same chain of methods as for the b < a and c < b comparisons.

If you write any of the following:

a in {d: 5}
a in set([d, d, d])
set([a, b]) == set([a, b])

Python first compares hashes, e.g.:

a.__hash__()
d.__hash__()

If hash(a) != hash(d) then the result of the statement a in {d: 5} is immediately False (remember how hashes work in general). If hash(a) == hash(d)) Python goes through the method resolution of the == operator as shown above.

General Notes and Caveats

In the method resolution for <, <=, ==, !=, >=, > and sorted([a, b, c]) operators the __hash__() method is not called, so in these cases it doesn’t matter what it returns. The __hash__() method is only called for sets and dictionaries.

In the official Python documentation you can read about hashable and non-hashable objects. In reality, you don’t have to think about it, you just follow the method resolution described here. E.g. if you try to use lists as dictionary keys, the list’s __hash__() method will be called and it returns an exception.

In SymPy, every instance of any subclass of Basic is immutable. Technically this means, that it’s behavior through all the methods above mustn’t change once the instance is created. Especially, the hash value mustn’t change (as already stated above) or else objects will get mixed up in dictionaries and wrong values will be returned for a given key, etc....

Script To Verify This Guide

The above method resolution can be verified using the following program:

class A(object):

    def __init__(self, a, hash):
        self.a = a
        self._hash = hash

    def __lt__(self, o):
        print "%s.__lt__(%s)" % (self.a, o.a)
        return NotImplemented

    def __le__(self, o):
        print "%s.__le__(%s)" % (self.a, o.a)
        return NotImplemented

    def __gt__(self, o):
        print "%s.__gt__(%s)" % (self.a, o.a)
        return NotImplemented

    def __ge__(self, o):
        print "%s.__ge__(%s)" % (self.a, o.a)
        return NotImplemented

    def __cmp__(self, o):
        print "%s.__cmp__(%s)" % (self.a, o.a)
        #return cmp(self._hash, o._hash)
        return NotImplemented

    def __eq__(self, o):
        print "%s.__eq__(%s)" % (self.a, o.a)
        return NotImplemented

    def __ne__(self, o):
        print "%s.__ne__(%s)" % (self.a, o.a)
        return NotImplemented

    def __hash__(self):
        print "%s.__hash__()" % (self.a)
        return self._hash

def show(s):
    print "--- %s " % s + "-"*40
    eval(s)

a = A("a", 1)
b = A("b", 2)
c = A("c", 3)
d = A("d", 1)

show("a == b")
show("a != b")
show("a < b")
show("a <= b")
show("a > b")
show("a >= b")
show("sorted([a, b, c])")
show("{d: 5}")
show("a in {d: 5}")
show("set([d, d, d])")
show("a in set([d, d, d])")
show("set([a, b])")

print "--- x = set([a, b]); y = set([a, b]); ---"
x = set([a,b])
y = set([a,b])
print "               x == y :"
x == y

print "--- x = set([a, b]); y = set([b, d]); ---"
x = set([a,b])
y = set([b,d])
print "               x == y :"
x == y

and its output:

--- a == b ----------------------------------------
a.__eq__(b)
b.__eq__(a)
a.__cmp__(b)
b.__cmp__(a)
--- a != b ----------------------------------------
a.__ne__(b)
b.__ne__(a)
a.__cmp__(b)
b.__cmp__(a)
--- a < b ----------------------------------------
a.__lt__(b)
b.__gt__(a)
a.__cmp__(b)
b.__cmp__(a)
--- a <= b ----------------------------------------
a.__le__(b)
b.__ge__(a)
a.__cmp__(b)
b.__cmp__(a)
--- a > b ----------------------------------------
a.__gt__(b)
b.__lt__(a)
a.__cmp__(b)
b.__cmp__(a)
--- a >= b ----------------------------------------
a.__ge__(b)
b.__le__(a)
a.__cmp__(b)
b.__cmp__(a)
--- sorted([a, b, c]) ----------------------------------------
b.__lt__(a)
a.__gt__(b)
b.__cmp__(a)
a.__cmp__(b)
c.__lt__(b)
b.__gt__(c)
c.__cmp__(b)
b.__cmp__(c)
--- {d: 5} ----------------------------------------
d.__hash__()
--- a in {d: 5} ----------------------------------------
d.__hash__()
a.__hash__()
d.__eq__(a)
a.__eq__(d)
d.__cmp__(a)
a.__cmp__(d)
--- set([d, d, d]) ----------------------------------------
d.__hash__()
d.__hash__()
d.__hash__()
--- a in set([d, d, d]) ----------------------------------------
d.__hash__()
d.__hash__()
d.__hash__()
a.__hash__()
d.__eq__(a)
a.__eq__(d)
d.__cmp__(a)
a.__cmp__(d)
--- set([a, b]) ----------------------------------------
a.__hash__()
b.__hash__()
--- x = set([a, b]); y = set([a, b]); ---
a.__hash__()
b.__hash__()
a.__hash__()
b.__hash__()
               x == y :
--- x = set([a, b]); y = set([b, d]); ---
a.__hash__()
b.__hash__()
b.__hash__()
d.__hash__()
               x == y :
d.__eq__(a)
a.__eq__(d)
d.__cmp__(a)
a.__cmp__(d)

[1]

There is also the similar NotImplementedError exception, which one may be tempted to raise to obtain the same effect as returning NotImplemented.

But these are not the same, and Python will completely ignore NotImplementedError with respect to choosing appropriate comparison method, and will just propagate this exception upwards, to the caller.

So return NotImplemented is not the same as raise NotImplementedError.

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