tutorials/GitHub_MD_rendering/Equality vs. identity.ipynb

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   "source": [
    "# Introduction\n",
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    "\n",
    "**What?** Equality vs. identity\n",
    "\n",
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    "\n",
    "# Idendity `is` always implies equality `==`?\n",
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    "- This oddity occurs, because Python keeps an array of small integer objects (i.e., integers between -5 and 256.\n",
    "- This is because the specific implementation of python based on Cypython reuses objects for small integers "
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      "a is b True\n",
      "c is d False\n"
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    "a = 1\n",
    "b = 1\n",
    "print('a is b', bool(a is b))\n",
    "True\n",
    "\n",
    "c = 999\n",
    "d = 999\n",
    "print('c is d', bool(c is d))"
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    "- This is in fact a CPython artifact and **must not necessarily be true** in all implementations of Python!\n",
    "- So the take home message is: always use \"==\" for equality, \"is\" for identity!\n",
    "- This example demonstrates that this applies  indeed for integers in the range in -5 to 256:"
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      "256 is 257-1 True\n",
      "257 is 258-1 False\n",
      "-5 is -6+1 True\n",
      "-7 is -6-1 False\n"
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    "print('256 is 257-1', 256 is 257-1)\n",
    "print('257 is 258-1', 257 is 258 - 1)\n",
    "print('-5 is -6+1', -5 is -6+1)\n",
    "print('-7 is -6-1', -7 is -6-1)"
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      "a is b, False\n",
      "a == b, True\n"
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    "a = 'hello world!'\n",
    "b = 'hello world!'\n",
    "print('a is b,', a is b)\n",
    "print('a == b,', a == b)"
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   "source": [
    "We would think that identity would always imply equality, but this is not always true, as we can see in the next example:"
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      "a is a, True\n",
      "a == a, False\n"
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    "a = float('nan')\n",
    "print('a is a,', a is a)\n",
    "print('a == a,', a == a)"
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   "source": [
    "# References\n",
    "<hr style = \"border:2px solid black\" ></hr>"
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   "source": [
    "\n",
    "- https://nbviewer.org/github/rasbt/python_reference/blob/master/tutorials/not_so_obvious_python_stuff.ipynb?create=1\n",
    "- https://docs.python.org/2/c-api/int.html#PyInt_FromLong\n",
    "- http://python.net/%7Egoodger/projects/pycon/2007/idiomatic/handout.html#other-languages-have-variables\n",
    "    \n",
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