4.2.1.Binding of names¶

Names refer to objects. Names are introduced by name binding operations.

The following constructs bind names:

• formal parameters to functions,

• class definitions,

• function definitions,

• assignment expressions,

• targets that are identifiers if occurring inan assignment:

  • for loop header,

  • afteras in awith statement,exceptclause,except* clause, or in the as-pattern in structural pattern matching,

  • in a capture pattern in structural pattern matching

• import statements.

• type statements.

• type parameter lists.

Theimport statement of the formfrom...import* binds allnames defined in the imported module, except those beginning with an underscore.This form may only be used at the module level.

A target occurring in adel statement is also considered bound forthis purpose (though the actual semantics are to unbind the name).

Each assignment or import statement occurs within a block defined by a class orfunction definition or at the module level (the top-level code block).

If a name is bound in a block, it is a local variable of that block, unlessdeclared asnonlocal orglobal. If a name is bound atthe module level, it is a global variable. (The variables of the module codeblock are local and global.) If a variable is used in a code block but notdefined there, it is afree variable.

Each occurrence of a name in the program text refers to thebinding ofthat name established by the following name resolution rules.

4.2.2.Resolution of names¶

Ascope defines the visibility of a name within a block. If a localvariable is defined in a block, its scope includes that block. If thedefinition occurs in a function block, the scope extends to any blocks containedwithin the defining one, unless a contained block introduces a different bindingfor the name.

When a name is used in a code block, it is resolved using the nearest enclosingscope. The set of all such scopes visible to a code block is called the block’senvironment.

When a name is not found at all, aNameError exception is raised.If the current scope is a function scope, and the name refers to a localvariable that has not yet been bound to a value at the point where the name isused, anUnboundLocalError exception is raised.UnboundLocalError is a subclass ofNameError.

If a name binding operation occurs anywhere within a code block, all uses of thename within the block are treated as references to the current block. This canlead to errors when a name is used within a block before it is bound. This ruleis subtle. Python lacks declarations and allows name binding operations tooccur anywhere within a code block. The local variables of a code block can bedetermined by scanning the entire text of the block for name binding operations.Seethe FAQ entry on UnboundLocalErrorfor examples.

If theglobal statement occurs within a block, all uses of the namesspecified in the statement refer to the bindings of those names in the top-levelnamespace. Names are resolved in the top-level namespace by searching theglobal namespace, i.e. the namespace of the module containing the code block,and the builtins namespace, the namespace of the modulebuiltins. Theglobal namespace is searched first. If the names are not found there, thebuiltins namespace is searched next. If the names are also not found in thebuiltins namespace, new variables are created in the global namespace.The global statement must precede all uses of the listed names.

Theglobal statement has the same scope as a name binding operationin the same block. If the nearest enclosing scope for a free variable containsa global statement, the free variable is treated as a global.

Thenonlocal statement causes corresponding names to referto previously bound variables in the nearest enclosing function scope.SyntaxError is raised at compile time if the given name does notexist in any enclosing function scope.Type parameterscannot be rebound with thenonlocal statement.

The namespace for a module is automatically created the first time a module isimported. The main module for a script is always called__main__.

Class definition blocks and arguments toexec() andeval() arespecial in the context of name resolution.A class definition is an executable statement that may use and define names.These references follow the normal rules for name resolution with an exceptionthat unbound local variables are looked up in the global namespace.The namespace of the class definition becomes the attribute dictionary ofthe class. The scope of names defined in a class block is limited to theclass block; it does not extend to the code blocks of methods. This includescomprehensions and generator expressions, but it does not includeannotation scopes,which have access to their enclosing class scopes.This means that the following will fail:

classA:a=42b=list(a+iforiinrange(10))

However, the following will succeed:

classA:typeAlias=NestedclassNested:passprint(A.Alias.__value__)# <type 'A.Nested'>

4.2.3.Annotation scopes¶

Type parameter lists andtype statementsintroduceannotation scopes, which behave mostly like function scopes,but with some exceptions discussed below.Annotationscurrently do not use annotation scopes, but they are expected to useannotation scopes in Python 3.13 whenPEP 649 is implemented.

Annotation scopes are used in the following contexts:

• Type parameter lists forgeneric type aliases.

• Type parameter lists forgeneric functions.A generic function’s annotations areexecuted within the annotation scope, but its defaults and decorators are not.

• Type parameter lists forgeneric classes.A generic class’s base classes andkeyword arguments are executed within the annotation scope, but its decorators are not.

• The bounds, constraints, and default values for type parameters(lazily evaluated).

• The value of type aliases (lazily evaluated).

Annotation scopes differ from function scopes in the following ways:

• Annotation scopes have access to their enclosing class namespace.If an annotation scope is immediately within a class scope, or within anotherannotation scope that is immediately within a class scope, the code in theannotation scope can use names defined in the class scope as if it wereexecuted directly within the class body. This contrasts with regularfunctions defined within classes, which cannot access names defined in the class scope.

• Expressions in annotation scopes cannot containyield,yieldfrom,await, or:=expressions. (These expressions are allowed in other scopes contained within theannotation scope.)

• Names defined in annotation scopes cannot be rebound withnonlocalstatements in inner scopes. This includes only type parameters, as no othersyntactic elements that can appear within annotation scopes can introduce new names.

• While annotation scopes have an internal name, that name is not reflected in thequalified name of objects defined within the scope.Instead, the__qualname__of such objects is as if the object were defined in the enclosing scope.

4.2.4.Lazy evaluation¶

The values of type aliases created through thetype statement arelazily evaluated. The same applies to the bounds, constraints, and default values of typevariables created through thetype parameter syntax.This means that they are not evaluated when the type alias or type variable iscreated. Instead, they are only evaluated when doing so is necessary to resolvean attribute access.

Example:

>>>typeAlias=1/0>>>Alias.__value__Traceback (most recent call last):...ZeroDivisionError:division by zero>>>deffunc[T:1/0]():pass>>>T=func.__type_params__[0]>>>T.__bound__Traceback (most recent call last):...ZeroDivisionError:division by zero

Here the exception is raised only when the__value__ attributeof the type alias or the__bound__ attribute of the type variableis accessed.

This behavior is primarily useful for references to types that have notyet been defined when the type alias or type variable is created. For example,lazy evaluation enables creation of mutually recursive type aliases:

fromtypingimportLiteraltypeSimpleExpr=int|ParenthesizedtypeParenthesized=tuple[Literal["("],Expr,Literal[")"]]typeExpr=SimpleExpr|tuple[SimpleExpr,Literal["+","-"],Expr]

Lazily evaluated values are evaluated inannotation scope,which means that names that appear inside the lazily evaluated value are looked upas if they were used in the immediately enclosing scope.

4.2.5.Builtins and restricted execution¶

The builtins namespace associated with the execution of a code blockis actually found by looking up the name__builtins__ in itsglobal namespace; this should be a dictionary or a module (in thelatter case the module’s dictionary is used). By default, when in the__main__ module,__builtins__ is the built-in modulebuiltins; when in any other module,__builtins__ is analias for the dictionary of thebuiltins module itself.

4.2.6.Interaction with dynamic features¶

Name resolution of free variables occurs at runtime, not at compile time.This means that the following code will print 42:

i=10deff():print(i)i=42f()

Theeval() andexec() functions do not have access to the fullenvironment for resolving names. Names may be resolved in the local and globalnamespaces of the caller. Free variables are not resolved in the nearestenclosing namespace, but in the global namespace.[1] Theexec() andeval() functions have optional arguments to override the global and localnamespace. If only one namespace is specified, it is used for both.