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4.2.1. Binding of names¶

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

When you run a single Python script from the command-line with python script.py, interpretation starts at the first line and continues line by line. If a line starts a class or function definition, the definition is stored for later reference. If the line is executable code, it is directly executed. When you run a single Python script from the command-line with python script.py, interpretation starts at the first line and continues line by line. If a line starts a class or function definition, the definition is stored for later reference. If the line is executable code, it is directly executed. Flow of Execution of the for Loop¶ As a program executes, the interpreter always keeps track of which statement is about to be executed. We call this the control flow, or the flow of execution of the program. When humans execute programs, they often use their finger to point to each statement in turn. Flow of Execution Summary¶. When you are working with functions it is really important to know the order in which statements are executed. This is called the flow of execution or control flow and we’ve already talked about it a number of times so far. To recap:. Execution always begins at the first statement of the program.

The following constructs bind names: formal parameters to functions,import statements, class and function definitions (these bind theclass or function name in the defining block), and targets that are identifiersif occurring in an assignment, for loop header, or afteras in a with statement or except clause.The import statementof the form from...import* binds all names defined in the importedmodule, except those beginning with an underscore. This form may only be usedat the module level.

A target occurring in a del 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 as nonlocal or global. 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 a free variable.

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

A program’scontrolflow is the orderin which the program’s code executes. The controlflow of a Python program is regulated by conditional statements,loops, and function calls. This section covers theif statement and for andwhile loops; functions are covered later in thischapter. Raising and handling exceptions also affects control flow;exceptions are covered in Chapter 6.

Often, you need to execute somestatements only if some condition holds, or choose statements toexecute depending on several mutually exclusive conditions. ThePython compound statement if, which usesif, elif, andelse clauses, lets you conditionally executeblocks of statements. Here’s the syntax for theif statement:

Theelif and else clauses areoptional. Note that unlike some languages, Python does not have aswitch statement, so you must useif, elif, andelse for all conditional processing.

Here’s a typical if statement:

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When there aremultiple statements in a clause (i.e., the clause controls a block ofstatements), the statements are placed on separate logical linesafter the line containing the clause’s keyword(known as the headerlineof the clause) and indented rightward from the header line. The blockterminates when the indentation returns to that of the clause header(or further left from there). When there is just a single simplestatement, as here, it can follow the : on thesame logical line as the header, but it can also be placed on aseparate logical line, immediately after the header line and indentedrightward from it. Many Python practitioners consider theseparate-line style more readable:

You can use any Python expression asthe condition in an if or elifclause. When you use an expression this way, you are using it in aBooleancontext. In aBoolean context, any value is taken as either true or false. As wediscussed earlier, any non-zero number or non-empty string, tuple,list, or dictionary evaluates as true. Zero (of any numeric type),None, and empty strings, tuples, lists, anddictionaries evaluate as false. When you want to test a valuex in a Boolean context, use the followingcoding style:

This is the clearest and most Pythonic form. Don’tuse:

There is a crucial difference between saying that an expression“returns True'(meaning the expression returns the value 1intended as a Boolean result) and saying that an expression“evaluates as true” (meaning theexpression returns any result that is true in a Boolean context).When testing an expression, you care about the latter condition, notthe former.

If the expression for the if clause evaluates astrue, the statements following the if clauseexecute, and the entire if statement ends.Otherwise, the expressions for any elif clausesare evaluated in order. The statements following the firstelif clause whose condition is true, if any, areexecuted, and the entire if statement ends.Otherwise, if an else clause exists, thestatements following it are executed.

The whilestatement in Python supports repeated execution of a statement orblock of statements that is controlled by a conditional expression.Here’s the syntax for the whilestatement:

A while statement can also include anelse clause and break andcontinue statements, as we’lldiscuss shortly.

Here’s a typical while statement:

First, expression, which is known as theloopcondition, isevaluated. If the condition is false, the whilestatement ends. If the loop condition is satisfied, the statement orstatements that comprise the loopbody are executed. When the loop body finishesexecuting, the loop condition is evaluated again, to see if anotheriteration should be performed. This process continues until the loopcondition is false, at which point the whilestatement ends.

The loop body should contain code that eventually makes the loopcondition false, or the loop will never end unless an exception israised or the loop body executes a breakstatement. A loop that is in a function’s body alsoends if a return statement executes in the loopbody, as the whole function ends in this case.

The for statementin Python supports repeated execution of a statement or block ofstatements that is controlled by an iterable expression.Here’s the syntax for the forstatement:

Note that the in keyword is part of the syntax ofthe for statement and is functionally unrelated tothe in operator used for membership testing. Afor statement can also include anelse clause and break andcontinue statements, as we’lldiscuss shortly.

Here’s a typical for statement:

iterable may be any Python expressionsuitable as an argument to built-in function iter,which returns an iterator object (explained in detail in the nextsection). target i