Use __slots__ to reduce Statement memory usage

[glennmatthews]

Use __slots__ Python language feature to reduce the memory overhead of each Statement object. Also serves as a handy way to list all of the many possible Statement instance attributes in one place. In my testing this can reduce pyang's memory overhead by about 25%.

Memory usage of my app using baseline pyang without this change:

(Pdb) from guppy import hpy
(Pdb) hpy().heap()
Partition of a set of 1569817 objects. Total size = 390492928 bytes.
 Index  Count   %     Size   % Cumulative  % Kind (class / dict of class)
     0  99535   6 156793960  40 156793960  40 dict of pyang.statements.Statement
     1 665307  42 76017560  19 232811520  60 list
     2  69021   4 37975224  10 270786744  69 dict (no owner)
     3  99555   6 27875400   7 298662144  76 dict of pyang.error.Position
     4  21854   1 23291824   6 321953968  82 collections.OrderedDict
     5  96418   6 15708544   4 337662512  86 unicode
     6 114464   7 11252864   3 348915376  89 str
     7  99555   6  6371520   2 355286896  91 pyang.error.Position
     8  99535   6  6370240   2 361657136  93 pyang.statements.Statement
     9  21854   1  6119120   2 367776256  94 dict of collections.OrderedDict

With this change:

(Pdb) from guppy import hpy
(Pdb) hpy().heap()
Partition of a set of 1564999 objects. Total size = 309499680 bytes.
 Index  Count   %     Size   % Cumulative  % Kind (class / dict of class)
     0 665306  43 76018000  25  76018000  25 list
     1  99535   6 55739600  18 131757600  43 pyang.statements.Statement
     2  69020   4 37975328  12 169732928  55 dict (no owner)
     3  99555   6 27875400   9 197608328  64 dict of pyang.error.Position
     4  94177   6 26369560   9 223977888  72 dict of pyang.statements.Statement
     5  21854   1 23291824   8 247269712  80 collections.OrderedDict
     6  96418   6 15708544   5 262978256  85 unicode
     7 114900   7 11293768   4 274272024  89 str
     8  99555   6  6371520   2 280643544  91 pyang.error.Position
     9  21854   1  6119120   2 286762664  93 dict of collections.OrderedDict

[glennmatthews]

Adding __slots__ to Position as well also takes it out of the top list:

Partition of a set of 1465357 objects. Total size = 283202912 bytes.
 Index  Count   %     Size   % Cumulative  % Kind (class / dict of class)
     0 665306  45 76018064  27  76018064  27 list
     1  99535   7 55739600  20 131757664  47 pyang.statements.Statement
     2  69020   5 37974560  13 169732224  60 dict (no owner)
     3  94177   6 26369560   9 196101784  69 dict of pyang.statements.Statement
     4  21854   1 23291824   8 219393608  77 collections.OrderedDict
     5  96416   7 15708304   6 235101912  83 unicode
     6 114859   8 11291352   4 246393264  87 str
     7  99555   7  7964400   3 254357664  90 pyang.error.Position
     8  21854   1  6119120   2 260476784  92 dict of collections.OrderedDict
     9  65195   4  6029160   2 266505944  94 tuple

[glennmatthews]

"make test" is passing - the Travis failure is the following, which I don't understand:

$ git ls-files . --exclude-standard --others | tee "$TMP"
.pytest_cache/v/cache/nodeids

The command "git ls-files . --exclude-standard --others | tee "$TMP"" exited with 0.

$ if test -s "$TMP"; then false; else true; fi

The command "if test -s "$TMP"; then false; else true; fi" exited with 1.

Any assistance would be appreciated.

[glennmatthews]

Possibly failure may be due to pytest-dev/pytest#3286.

[mbj4668]

Thanks for this PR! I'll play with this locally before merging.

[glennmatthews]

Thank you Martin!