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Reference count checking (Python recipe) by Robin Becker
ActiveState Code (http://code.activestate.com/recipes/496791/)

Whilst testing that a C extension was causing a problem with reference counts I created some code to check that the reference counts of a set of variables were not being changed by a call to a specific C method. That code itself was buggy as the output of a particular reference count eg 12 could actually modify one or more of the reference counts being checked. The solution was to store the refcount info as a string.

Python, 67 lines
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def getrc(defns,depth=1):
    '''get rc's for things in the space separated string defns'''
    from sys import getrefcount, _getframe
    f = _getframe(depth)
    G0 = f.f_globals
    L = f.f_locals
    if L is not G0:
        LL = [L]
        while 1:
            f = f.f_back
            G = f.f_globals
            L = f.f_locals
            if G is not G0 or G is L: break
            LL.append(L)
        L = {}
        for l in reversed(LL):
            L.update(l)
    else:
        L = L.copy()
    G0 = G0.copy()
    return ' '.join([str(getrefcount(eval(x,L,G0))-1) for x in defns.split()])

def checkrc(defns,rcv0):
    '''check rc's of things in defns against previously stored values
    return a string indicating those that have changed
    '''
    rcv1 = getrc(defns,2)
    return ' '.join(["%s %s-->%s" % (x,v,w) for x,v,w in zip(defns.split(),rcv0.split(),rcv1.split()) if v!=w])

def testStringWidth(incFontSize):
    '''example from the reportlab test set'''
    from _rl_accel import stringWidthU
    from reportlab.pdfbase.pdfmetrics import _py_stringWidth, getFont, registerFont, _fonts 
    from reportlab.pdfbase.ttfonts import TTFont
    ttfn = 'Luxi-Serif'
    t1fn = 'Times-Roman'
    registerFont(TTFont(ttfn, "luxiserif.ttf"))
    ttf = getFont(ttfn)
    t1f = getFont(t1fn)
    testCp1252 = 'copyright %s trademark %s registered %s ReportLab! Ol%s!' % (chr(169), chr(153),chr(174), chr(0xe9))
    enc='cp1252'
    senc = 'utf8'
    intern(senc)
    ts = 'ABCDEF\xce\x91\xce\xb2G'
    utext = 'ABCDEF\xce\x91\xce\xb2G'.decode('utf8')
    fontSize = 12
    defns="ttfn t1fn ttf t1f testCp1252 enc senc ts utext fontSize ttf.face ttf.face.charWidths ttf.face.defaultWidth t1f.widths t1f.encName t1f.substitutionFonts _fonts"
    rcv = getrc(defns)  #compute initial ref
    def tfunc(ts,fn,fontSize,enc):
        w1 = stringWidthU(ts,fn,fontSize,enc)
        w2 = _py_stringWidth(ts,fn,fontSize,enc)
        assert abs(w1-w2)<1e-10,"stringWidthU(%r,%r,%s,%r)-->%r != _py_stringWidth(...)-->%r" % (ts,fn,fontSize,enc,w1,w2)
    tfunc(testCp1252,t1fn,fontSize,enc)
    tfunc(ts,t1fn,fontSize,senc)
    tfunc(utext,t1fn,fontSize,senc)
    tfunc(ts,ttfn,fontSize,senc)
    tfunc(testCp1252,ttfn,fontSize,enc)
    tfunc(utext,ttfn,fontSize,senc)
    if incFontSize:
        z = fontSize    #simulate adding a reference to fontSize
    print checkrc(defns,rcv)

if __name__=='__main__':
    print 'should have no output',
    testStringWidth(0)
    print 'should change fontSize rc',
    testStringWidth(1)

I use this in standard unit tests of accelerator methods in the ReportLab toolkit. We already have to check the input output behaviour of the methods/functions, but there's an additional problem with C extensions eg they mustn't mess up the reference counts of things passed to them or which they use implicitly. An additional hazard which is not covered here is that of memory leaks. I can't find a simple way to test for that. Usually calling the functions a few thousand times and looking at the overall memory usage is enough.