7. MODULES. Rocky K. C. Chang October 17, (Based on Dierbach)

Transcription

1 7. MODULES Rocky K. C. Chang October 17, 2016 (Based on Dierbach)

2 Objectives Explain the specification of modules. Become familiar with the use of docstrings in Python. Explain the use of modules and namespaces in Python. Describe and use the different forms of module import in Python. Explain the local, global, and built-in namespaces and the scope resolution.

3 Where do the modules come from? The term module refers to the design and/or implementation of specific functionality to be incorporated into a program. Modular-II MODULE Hello; FROM STextIO IMPORT WriteString; BEGIN WriteString("Hello World!"); END Hello. A Python module is a file containing Python definitions and statements. By convention, modules are named using all lower case letters and optional underscore characters.

4 EXERCISE 7.1 Import a module. Use help(module_name) or dir(module_name) to find out the functions available in that module. Use print(module_name) to find out what the module does. Use print(function_name. doc ) to print out what the function does.

5 Interface, client and docstring A module s interface is a specification of what it provides and how it is to be used. Any program code making use of a given module is called a client of the module. A docstring is a string literal denoted by triple quotes used in Python for providing the specification of certain program elements.

6 EXERCISE 7.2 Try Import the graphics module dir(graphics) help(graphics) print(graphics. doc )

7 Documentation String (docstring) ( A docstring is a string literal denoted by triple quotes given as the first line of certain program elements. These additional lines must be indented at the same level. The docstring for a module is simply the first block of quoted text to appear in the module. Write docstrings for all public modules, functions, classes, and methods. Convention: """Return a foobang Optional plotz says to frobnicate the bizbaz first. """

8 EXERCISE 7.3 Write docstrings for your previous assignment.

9 EXERCISE 7.4 Run graphics.py as a main module. Run graphics.py as an imported module, for example, using your code for exercise What is the difference between the two?

10 Running a module as main or imported module Before executing the code, the Python interpreter will define a few special variables, e.g., name. If the python interpreter is running that module (the source file) as the main program, it sets the special name variable to have a value " main ". If this file is being imported from another module, name will be set to the module's name. test() in graphics.py will be executed only when the module is run as main: if name == " main ": test() See main.html

11 EXERCISE 7.5 Create a simple module a (e.g., just printing out a message) and put it in your folder for modules (not in the Lib folder under Python-3.4.1) Create another simple module b that imports module a. Run module b.

12 Searching for imported modules Each imported module of a Python program needs to be located and loaded into memory. Python first searches for modules in the current directory. If the module is not found, it searches the directories specified in the PYTHONPATH environment variable. If the module is still not found, a Python installationspecific path is searched (e.g., C:\Python32\Lib). If the program still does not find the module, an ImportError exception is reported.

13 The main and imported modules The main module may import any number of other modules (and each of those modules import other modules, etc.). Main modules are not meant to be imported into other modules. The statements of imported modules are executed only once, the first time that the module is imported. When a module is loaded, a compiled version of the module with file extension.pyc is automatically produced. Imported modules: Standard library (usually written in C) or Other modules (usually written in Python)

14 EXERCISE 7.6 Try the code on the next page. What will be printed out? Next, comment x=1 inside funa() and run it again. Now, add x = 10 before x = funa() and run it again.

15 # A module def funa(): # x is a local variable x = 1 print(x) return x def funb(): # x is a local variable x = 2 print(x) return x # x is a global variable x = funa() x = funb() x = 0 print(x)

16 Global and local variables A local variable is a variable that is only accessible from within the function it resides. Such variables are said to have local scope. A global variable is a variable defined outside of any function definition. Such variables are said to have global scope. The use of global variables is considered bad programming practice.

17 Scope resolution Scope of a name (identifier) is the set of program statements over which it can be referred to. The scope of a name depends on where it is created and whether the name is used somewhere else. Local and global scopes When encountering a name, Python interpreter will search it in the order below: Local Enclosing (don't worry about it) Global Built-in

18 EXERCISE 7.7 Run the code below and try to explain the result. x = 1 def fun(): x = x + 1 print(x, "x inside fun()") print(x, "x outside fun()") fun()

19 The local assignment rule If anywhere in a function an assignment is made, then that assignment is assumed to create a name only in the presently active namespace. If, within a function, a variable is declared to be a global variable using the global statement, then Python will not create a local name in the namespace for that variable.

20 Modules and Namespaces A namespace is a container that provides a named context for a set of identifiers (for variables, functions, and constants). A name clash is when two otherwise distinct entities with the same name become part of the same scope. In Python, each module has its own namespace. This includes the names of all items in the module. The fully qualified name of each identifier in a module is of the form modulename.identifier. math.sqrt() in a (e.g., math) module is used when the module is imported.

21 EXERCISE 7.8 Use the interactive mode for this exercise. 1. Restart the shell. 2. Type dir(). 3. Run the module in Exercise Type dir(). 5. Repeat steps 1-4 for graphics.py.

22 A module's/global namespace In Python, the main module of any program is the first ( top-level ) module executed. When working interactively in the Python shell, the Python interpreter functions as the main module, containing the global namespace. The namespace is reset every time the interpreter is started. Six names are always there: ' builtins ', ' doc ', ' loader ', ' name ', ' package ', ' spec '

23 EXERCISE 7.9 In the shell used for the last exercise, import another module and then find out the updated namespace by dir(). Consider the following four cases: import math from math import sqrt, log from math import sqrt as new_sqrt from math import *

24 Importing modules When using import module_name, the namespace of the imported module becomes available to, but not part of, the importing module. The imported identifiers must be referenced fully qualified. When using from module_name import something, the imported module s namespace becomes part of the importing module s namespace. The imported identifiers are referenced without being fully qualified.

25 From-import Three possible from-imports: from module_name import func1, func2 from module_name import func1 as new_func1 from module_name import * For import *, All of the identifiers are imported, except for those that begin with two underscore characters, which are meant to be private in the module. In Python, all the variables in a module are public, with the convention that variables beginning with two underscores are intended to be private.

26 EXERCISE 7.10 Try from math import * Define a new function: def ceil(x): if x - int(x) >= 0.5: return int(x)+1 else: return int(x) Execute ceil() to see which ceil() is used. Execute math.ceil().

27 Local, Global, and Built-in Namespaces During a Python program s execution, there are as many as three namespaces: The built-in namespace contains the names of all the built-in functions, constants, and so on, in Python. The global namespace contains the identifiers of the currently executing module. The local namespace is the namespace of the currently executing function (if any).

28 EXERCISE 7.11 Run the module in the next page. Try to understand what the outputs tell you.

29 import math globalx = 100 def getlocalnamespace(p1=123, p2="hihi"): localx = 10 print("\n=== local namespace ===") for key, val in locals().items(): print("key: {0:10} object: {1:10}".format(key, str(val))) def getglobalnamespace(): print("\n=== global namespace ===") for key, val in globals().items(): print("key: {0:20} object: {1:20}".format(key, str(val))) getlocalnamespace() getglobalnamespace()

30 locals() and globals() built-in functions locals(): Update and return a dictionary representing the current local symbol table. globals(): Return a dictionary representing the current global symbol table.

31 EXERCISE 7.12 Run the module in the next page. Try to understand what the outputs tell you.

32 def getbuiltins(): builtin_dict = builtins. dict print("\nbuiltin dictionary has {} entries\n".format(len(builtin_dict))) for key, val in builtin_dict.items(): print("key: {0:20} object: {1:20}".format(key, str(val))) getbuiltins()

33 The builtins module When the Python interpreter starts up, it loads two default modules without requiring an import: main and builtins. The builtins module provides the built-in functions and built-in constants. More details on this module:

34 END