(window.webpackJsonp=window.webpackJsonp||[]).push([[2839],{3247:function(t,s,a){"use strict";a.r(s);var n=a(31),e=Object(n.a)({},(function(){var t=this,s=t.$createElement,a=t._self._c||s;return a("ContentSlotsDistributor",{attrs:{"slot-key":t.$parent.slotKey}},[a("h1",{attrs:{id:"variable-scope-and-binding"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#variable-scope-and-binding"}},[t._v("#")]),t._v(" Variable Scope and Binding")]),t._v(" "),a("h2",{attrs:{id:"nonlocal-variables"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#nonlocal-variables"}},[t._v("#")]),t._v(" Nonlocal Variables")]),t._v(" "),a("p",[t._v("Python 3 added a new keyword called "),a("strong",[t._v("nonlocal")]),t._v(". The nonlocal keyword adds a scope override to the inner scope. You can read all about it in "),a("a",{attrs:{href:"https://www.python.org/dev/peps/pep-3104/",target:"_blank",rel:"noopener noreferrer"}},[t._v("PEP 3104"),a("OutboundLink")],1),t._v(". This is best illustrated with a couple of code examples. One of the most common examples is to create function that can increment:")]),t._v(" "),a("div",{staticClass:"language-py extra-class"},[a("pre",{pre:!0,attrs:{class:"language-py"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("def")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("counter")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v("\n    num "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("def")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("incrementer")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v("\n        num "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("1")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" num\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" incrementer\n\n")])])]),a("p",[t._v("If you try running this code, you will receive an "),a("strong",[t._v("UnboundLocalError")]),t._v(" because the "),a("strong",[t._v("num")]),t._v(" variable is referenced before it is assigned in the innermost function. Let's add nonlocal to the mix:")]),t._v(" "),a("div",{staticClass:"language-py extra-class"},[a("pre",{pre:!0,attrs:{class:"language-py"}},[a("code",[a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("def")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("counter")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v("\n    num "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("0")]),t._v("\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("def")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("incrementer")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("nonlocal")]),t._v(" num\n        num "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("+=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("1")]),t._v("\n        "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" num\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("return")]),t._v(" incrementer\n\nc "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" counter"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("\nc"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("# = 1")]),t._v("\nc"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("# = 2")]),t._v("\nc"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("# = 3")]),t._v("\n\n")])])]),a("p",[t._v("Basically "),a("code",[t._v("nonlocal")]),t._v(" will allow you to assign to variables in an outer scope, but not a global scope. So you can't use "),a("code",[t._v("nonlocal")]),t._v(" in our "),a("code",[t._v("counter")]),t._v(" function because then it would try to assign to a global scope. Give it a try and you will quickly get a "),a("code",[t._v("SyntaxError")]),t._v(". Instead you must use "),a("code",[t._v("nonlocal")]),t._v(" in a nested function.")]),t._v(" "),a("p",[t._v("(Note that the functionality presented here is better implemented using generators.)")]),t._v(" "),a("h2",{attrs:{id:"global-variables"}},[a("a",{staticClass:"header-anchor",attrs:{href:"#global-variables"}},[t._v("#")]),t._v(" Global Variables")]),t._v(" "),a("p",[t._v("In Python, variables inside functions are considered local if and only if they appear in the left side of an assignment statement, or some other binding occurrence; otherwise such a binding is looked up in enclosing functions, up to the global scope. 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"),a("span",{pre:!0,attrs:{class:"token function"}},[t._v("func")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(":")]),t._v("\n    foo "),a("span",{pre:!0,attrs:{class:"token operator"}},[t._v("=")]),t._v(" "),a("span",{pre:!0,attrs:{class:"token number"}},[t._v("2")]),t._v("  "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("# creates a new variable foo in local scope, global foo is not affected")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token keyword"}},[t._v("print")]),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v("(")]),t._v("foo"),a("span",{pre:!0,attrs:{class:"token punctuation"}},[t._v(")")]),t._v("  "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("# prints 2")]),t._v("\n\n    "),a("span",{pre:!0,attrs:{class:"token comment"}},[t._v("# global variable foo still exists, unchanged:")]),t._v("\n    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