Learning Python - Mark Lutz [157]
Immutable types. Define a string S of four characters again: S = "spam". Write an assignment that changes the string to "slam", using only slicing and concatenation. Could you perform the same operation using just indexing and concatenation? How about index assignment?
Nesting. Write a data structure that represents your personal information: name (first, middle, last), age, job, address, email address, and phone number. You may build the data structure with any combination of built-in object types you like (lists, tuples, dictionaries, strings, numbers). Then, access the individual components of your data structures by indexing. Do some structures make more sense than others for this object?
Files. Write a script that creates a new output file called myfile.txt and writes the string "Hello file world!" into it. Then write another script that opens myfile.txt and reads and prints its contents. Run your two scripts from the system command line. Does the new file show up in the directory where you ran your scripts? What if you add a different directory path to the filename passed to open? Note: file write methods do not add newline characters to your strings; add an explicit \n at the end of the string if you want to fully terminate the line in the file.
Part III. Statements and Syntax
Chapter 10. Introducing Python Statements
Now that you’re familiar with Python’s core built-in object types, this chapter begins our exploration of its fundamental statement forms. As in the previous part, we’ll begin here with a general introduction to statement syntax, and we’ll follow up with more details about specific statements in the next few chapters.
In simple terms, statements are the things you write to tell Python what your programs should do. If programs “do things with stuff,” statements are the way you specify what sort of things a program does. Python is a procedural, statement-based language; by combining statements, you specify a procedure that Python performs to satisfy a program’s goals.
Python Program Structure Revisited
Another way to understand the role of statements is to revisit the concept hierarchy introduced in Chapter 4, which talked about built-in objects and the expressions used to manipulate them. This chapter climbs the hierarchy to the next level:
Programs are composed of modules.
Modules contain statements.
Statements contain expressions.
Expressions create and process objects.
At its core, Python syntax is composed of statements and expressions. Expressions process objects and are embedded in statements. Statements code the larger logic of a program’s operation—they use and direct expressions to process the objects we studied in the preceding chapters. Moreover, statements are where objects spring into existence (e.g., in expressions within assignment statements), and some statements create entirely new kinds of objects (functions, classes, and so on). Statements always exist in modules, which themselves are managed with statements.
Python’s Statements
Table 10-1 summarizes Python’s statement set. This part of the book deals with entries in the table from the top through break and continue. You’ve informally been introduced to a few of the statements in Table 10-1 already; this part of the book will fill in details that were skipped earlier, introduce the rest of Python’s procedural statement set, and cover the overall syntax model. Statements lower in Table 10-1 that have to do with larger program units—functions, classes, modules, and exceptions—lead to larger programming ideas, so they will each have a section of their own. More focused statements (like del, which deletes various components) are covered elsewhere in the book, or in Python’s standard manuals.
Table 10-1. Python 3.0 statements
Statement
Role
Example
Assignment
Creating references
a, *b = 'good', 'bad', 'ugly'