Learning Python (2013)

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, as suggested in Chapter 4, programs “do things with stuff,” then statements are the way you specify what sort of things a program does. Less informally, Python is a procedural, statement-based language; by combining statements, you specify a procedure that Python performs to satisfy a program’s goals.

The Python Conceptual Hierarchy 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 of Python program structure:

1.    Programs are composed of modules.

2.    Modules contain statements.

3.    Statements contain expressions.

4.    Expressions create and process objects.

At their base, programs written in the Python language are 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). At the top, statements always exist in modules, which themselves are managed with statements.

Python’s Statements

Table 10-1 summarizes Python’s statement set. Each statement in Python has its own specific purpose and its own specific syntax—the rules that define its structure—though, as we’ll see, many share common syntax patterns, and some statements’ roles overlap. Table 10-1 also gives examples of each statement, when coded according to its syntax rules. In your programs, these units of code can perform actions, repeat tasks, make choices, build larger program structures, and so on.

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 statements

Statement

Role

Example

Assignment

Creating references

a, b = 'good', 'bad'

Calls and other expressions

Running functions

log.write("spam, ham")

print calls

Printing objects

print('The Killer', joke)

if/elif/else

Selecting actions

if "python" in text:

    print(text)

for/else

Iteration

for x in mylist:

    print(x)

while/else

General loops

while X > Y:

    print('hello')

pass

Empty placeholder

while True:

    pass

break

Loop exit

while True:

    if exittest(): break

continue

Loop continue

while True:

    if skiptest(): continue

def

Functions and methods

def f(a, b, c=1, *d):

    print(a+b+c+d[0])

return

Functions results

def f(a, b, c=1, *d):

    return a+b+c+d[0]

yield

Generator functions

def gen(n):

    for i in n: yield i*2

global

Namespaces

x = 'old'

def function():

    global x, y; x = 'new'

nonlocal

Namespaces (3.X)

def outer():

    x = 'old'

    def function():

        nonlocal x; x = 'new'

import

Module access

import sys

from

Attribute access

from sys import stdin

class

Building objects

class Subclass(Superclass):

    staticData = []

    def method(self): pass

try/except/ finally

Catching exceptions

try:

    action()

except:

    print('action error')

raise

Triggering exceptions

raise EndSearch(location)

assert

Debugging checks

assert X > Y, 'X too small'

with/as

Context managers (3.X, 2.6+)

with open('data') as myfile:

    process(myfile)

del

Deleting references

del data[k]

del data[i:j]

del obj.attr

del variable

Technically, Table 10-1 reflects Python 3.X’s statements. Though sufficient as a quick preview and reference, it’s not quite complete as is. Here are a few fine points about its content:

§  Assignment statements come in a variety of syntax flavors, described in Chapter 11: basic, sequence, augmented, and more.

§  print is technically neither a reserved word nor a statement in 3.X, but a built-in function call; because it will nearly always be run as an expression statement, though (and often on a line by itself), it’s generally thought of as a statement type. We’ll study print operations in Chapter 11.

§  yield is also an expression instead of a statement as of 2.5; like print, it’s typically used as an expression statement and so is included in this table, but scripts occasionally assign or otherwise use its result, as we’ll see in Chapter 20. As an expression, yield is also a reserved word, unlike print.

Most of this table applies to Python 2.X, too, except where it doesn’t—if you are using Python 2.X, here are a few notes for your Python, too:

§  In 2.X, nonlocal is not available; as we’ll see in Chapter 17, there are alternative ways to achieve this statement’s writeable state-retention effect.

§  In 2.X, print is a statement instead of a built-in function call, with specific syntax covered in Chapter 11.

§  In 2.X, the 3.X exec code execution built-in function is a statement, with specific syntax; since it supports enclosing parentheses, though, you can generally use its 3.X call form in 2.X code.

§  In 2.5, the try/except and try/finally statements were merged: the two were formerly separate statements, but we can now say both except and finally in the same try statement.

§  In 2.5, with/as is an optional extension, and it is not available unless you explicitly turn it on by running the statement from __future__ import with_statement (see Chapter 34).

A Tale of Two ifs

Before we delve into the details of any of the concrete statements in Table 10-1, I want to begin our look at Python statement syntax by showing you what you are not going to type in Python code so you can compare and contrast it with other syntax models you might have seen in the past.

Consider the following if statement, coded in a C-like language:

if (x > y) {

    x = 1;

    y = 2;

}

This might be a statement in C, C++, Java, JavaScript, or similar. Now, look at the equivalent statement in the Python language:

if x > y:

    x = 1

    y = 2

The first thing that may pop out at you is that the equivalent Python statement is less, well, cluttered—that is, there are fewer syntactic components. This is by design; as a scripting language, one of Python’s goals is to make programmers’ lives easier by requiring less typing.

More specifically, when you compare the two syntax models, you’ll notice that Python adds one new thing to the mix, and that three items that are present in the C-like language are not present in Python code.

What Python Adds

The one new syntax component in Python is the colon character (:). All Python compound statements—statements that have other statements nested inside them—follow the same general pattern of a header line terminated in a colon, followed by a nested block of code usually indented underneath the header line, like this:

Header line:

    Nested statement block

The colon is required, and omitting it is probably the most common coding mistake among new Python programmers—it’s certainly one I’ve witnessed thousands of times in Python training classes I’ve taught. In fact, if you are new to Python, you’ll almost certainly forget the colon character very soon. You’ll get an error message if you do, and most Python-friendly editors make this mistake easy to spot. Including it eventually becomes an unconscious habit (so much so that you may start typing colons in your C-like language code, too, generating many entertaining error messages from that language’s compiler!).

What Python Removes

Although Python requires the extra colon character, there are three things programmers in C-like languages must include that you don’t generally have to in Python.

Parentheses are optional

The first of these is the set of parentheses around the tests at the top of the statement:

if (x < y)

The parentheses here are required by the syntax of many C-like languages. In Python, though, they are not—we simply omit the parentheses, and the statement works the same way:

if x < y

Technically speaking, because every expression can be enclosed in parentheses, including them will not hurt in this Python code, and they are not treated as an error if present.

But don’t do that: you’ll be wearing out your keyboard needlessly, and broadcasting to the world that you’re a programmer of a C-like language still learning Python (I know, because I was once, too). The “Python way” is to simply omit the parentheses in these kinds of statements altogether.

End-of-line is end of statement

The second and more significant syntax component you won’t find in Python code is the semicolon. You don’t need to terminate statements with semicolons in Python the way you do in C-like languages:

x = 1;

In Python, the general rule is that the end of a line automatically terminates the statement that appears on that line. In other words, you can leave off the semicolons, and it works the same way:

x = 1

There are some ways to work around this rule, as you’ll see in a moment (for instance, wrapping code in a bracketed structure allows it to span lines). But, in general, you write one statement per line for the vast majority of Python code, and no semicolon is required.

Here, too, if you are pining for your C programming days (if such a state is possible) you can continue to use semicolons at the end of each statement—the language lets you get away with them if they are present, because the semicolon is also a separator when statements are combined.

But don’t do that either (really!). Again, doing so tells the world that you’re a programmer of a C-like language who still hasn’t quite made the switch to Python coding. The Pythonic style is to leave off the semicolons altogether. Judging from students in classes, this seems a tough habit for some veteran programmers to break. But you’ll get there; semicolons are useless noise in this role in Python.

End of indentation is end of block

The third and final syntax component that Python removes, and the one that may seem the most unusual to soon-to-be-ex-programmers of C-like languages (until they’ve used it for 10 minutes and realize it’s actually a feature), is that you do not type anything explicit in your code to syntactically mark the beginning and end of a nested block of code. You don’t need to include begin/end, then/endif, or braces around the nested block, as you do in C-like languages:

if (x > y) {

    x = 1;

    y = 2;

}

Instead, in Python, we consistently indent all the statements in a given single nested block the same distance to the right, and Python uses the statements’ physical indentation to determine where the block starts and stops:

if x > y:

    x = 1

    y = 2

By indentation, I mean the blank whitespace all the way to the left of the two nested statements here. Python doesn’t care how you indent (you may use either spaces or tabs), or how much you indent (you may use any number of spaces or tabs). In fact, the indentation of one nested block can be totally different from that of another. The syntax rule is only that for a given single nested block, all of its statements must be indented the same distance to the right. If this is not the case, you will get a syntax error, and your code will not run until you repair its indentation to be consistent.

Why Indentation Syntax?

The indentation rule may seem unusual at first glance to programmers accustomed to C-like languages, but it is a deliberate feature of Python, and it’s one of the main ways that Python almost forces programmers to produce uniform, regular, and readable code. It essentially means that you must line up your code vertically, in columns, according to its logical structure. The net effect is to make your code more consistent and readable (unlike much of the code written in C-like languages).

To put that more strongly, aligning your code according to its logical structure is a major part of making it readable, and thus reusable and maintainable, by yourself and others. In fact, even if you never use Python after reading this book, you should get into the habit of aligning your code for readability in any block-structured language. Python underscores the issue by making this a part of its syntax, but it’s an important thing to do in any programming language, and it has a huge impact on the usefulness of your code.

Your experience may vary, but when I was still doing development on a full-time basis, I was mostly paid to work on large old C++ programs that had been worked on by many programmers over the years. Almost invariably, each programmer had his or her own style for indenting code. For example, I’d often be asked to change a while loop coded in the C++ language that began like this:

while (x > 0) {

Before we even get into indentation, there are three or four ways that programmers can arrange these braces in a C-like language, and organizations often endure political battles and standards manuals to address the options (which seems more than a little off-topic for the problem to be solved by programming). Be that as it may, here’s the scenario I often encountered in C++ code. The first person who worked on the code indented the loop four spaces:

while (x > 0) {

    --------;

    --------;

That person eventually moved on to management, only to be replaced by someone who liked to indent further to the right:

while (x > 0) {

    --------;

    --------;

           --------;

           --------;

That person later moved on to other opportunities (ending that individual’s reign of coding terror...), and someone else picked up the code who liked to indent less:

while (x > 0) {

    --------;

    --------;

           --------;

           --------;

--------;

--------;

}

And so on. Eventually, the block is terminated by a closing brace (}), which of course makes this “block-structured code” (he says, sarcastically). No: in any block-structured language, Python or otherwise, if nested blocks are not indented consistently, they become very difficult for the reader to interpret, change, or reuse, because the code no longer visually reflects its logical meaning. Readability matters, and indentation is a major component of readability.

Here is another example that may have burned you in the past if you’ve done much programming in a C-like language. Consider the following statement in C:

if (x)

     if (y)

          statement1;

else

     statement2;

Which if does the else here go with? Surprisingly, the else is paired with the nested if statement (if (y)) in C, even though it looks visually as though it is associated with the outer if (x). This is a classic pitfall in the C language, and it can lead to the reader completely misinterpreting the code and changing it incorrectly in ways that might not be uncovered until the Mars rover crashes into a giant rock!

This cannot happen in Python—because indentation is significant, the way the code looks is the way it will work. Consider an equivalent Python statement:

if x:

     if y:

          statement1

else:

     statement2

In this example, the if that the else lines up with vertically is the one it is associated with logically (the outer if x). In a sense, Python is a WYSIWYG language—what you see is what you get—because the way code looks is the way it runs, regardless of who coded it.

If this still isn’t enough to underscore the benefits of Python’s syntax, here’s another anecdote. Early in my career, I worked at a successful company that developed systems software in the C language, where consistent indentation is not required. Even so, when we checked our code into source control at the end of the day, this company ran an automated script that analyzed the indentation used in the code. If the script noticed that we’d indented our code inconsistently, we received an automated email about it the next morning—and so did our managers!

The point is that even when a language doesn’t require it, good programmers know that consistent use of indentation has a huge impact on code readability and quality. The fact that Python promotes this to the level of syntax is seen by most as a feature of the language.

Also keep in mind that nearly every programmer-friendly text editor has built-in support for Python’s syntax model. In the IDLE Python GUI, for example, lines of code are automatically indented when you are typing a nested block; pressing the Backspace key backs up one level of indentation, and you can customize how far to the right IDLE indents statements in a nested block. There is no universal standard on this: four spaces or one tab per level is common, but it’s generally up to you to decide how and how much you wish to indent (unless you work at a company that’s endured politics and manuals to standardize this too). Indent further to the right for further nested blocks, and less to close the prior block.

As a rule of thumb, you probably shouldn’t mix tabs and spaces in the same block in Python, unless you do so consistently; use tabs or spaces in a given block, but not both (in fact, Python 3.X now issues an error for inconsistent use of tabs and spaces, as we’ll see in Chapter 12). Then again, you probably shouldn’t mix tabs or spaces in indentation in any structured language—such code can cause major readability issues if the next programmer has his or her editor set to display tabs differently than yours. C-like languages might let coders get away with this, but they shouldn’t: the result can be a mangled mess.

Regardless of which language you code in, you should be indenting consistently for readability. In fact, if you weren’t taught to do this earlier in your career, your teachers did you a disservice. Most programmers—especially those who must read others’ code—consider it a major asset that Python elevates this to the level of syntax. Moreover, generating tabs instead of braces is no more difficult in practice for tools that must output Python code. In general, if you do what you should be doing in a C-like language anyhow, but get rid of the braces, your code will satisfy Python’s syntax rules.

A Few Special Cases

As mentioned previously, in Python’s syntax model:

§  The end of a line terminates the statement on that line (without semicolons).

§  Nested statements are blocked and associated by their physical indentation (without braces).

Those rules cover almost all Python code you’ll write or see in practice. However, Python also provides some special-purpose rules that allow customization of both statements and nested statement blocks. They’re not required and should be used sparingly, but programmers have found them useful in practice.

Statement rule special cases

Although statements normally appear one per line, it is possible to squeeze more than one statement onto a single line in Python by separating them with semicolons:

a = 1; b = 2; print(a + b)               # Three statements on one line

This is the only place in Python where semicolons are required: as statement separators. This only works, though, if the statements thus combined are not themselves compound statements. In other words, you can chain together only simple statements, like assignments, prints, and function calls. Compound statements like if tests and while loops must still appear on lines of their own (otherwise, you could squeeze an entire program onto one line, which probably would not make you very popular among your coworkers!).

The other special rule for statements is essentially the inverse: you can make a single statement span across multiple lines. To make this work, you simply have to enclose part of your statement in a bracketed pair—parentheses (()), square brackets ([]), or curly braces ({}). Any code enclosed in these constructs can cross multiple lines: your statement doesn’t end until Python reaches the line containing the closing part of the pair. For instance, to continue a list literal:

mylist = [1111,

          2222,

          3333]

Because the code is enclosed in a square brackets pair, Python simply drops down to the next line until it encounters the closing bracket. The curly braces surrounding dictionaries (as well as set literals and dictionary and set comprehensions in 3.X and 2.7) allow them to span lines this way too, and parentheses handle tuples, function calls, and expressions. The indentation of the continuation lines does not matter, though common sense dictates that the lines should be aligned somehow for readability.

Parentheses are the catchall device—because any expression can be wrapped in them, simply inserting a left parenthesis allows you to drop down to the next line and continue your statement:

X = (A + B +

     C + D)

This technique works with compound statements, too, by the way. Anywhere you need to code a large expression, simply wrap it in parentheses to continue it on the next line:

if (A == 1 and

    B == 2 and

    C == 3):

        print('spam' * 3)

An older rule also allows for continuation lines when the prior line ends in a backslash:

X = A + B + \

      C + D                  # An error-prone older alternative

This alternative technique is dated, though, and is frowned on today because it’s difficult to notice and maintain the backslashes. It’s also fairly brittle and error-prone—there can be no spaces after the backslash, and accidentally omitting it can have unexpected effects if the next line is mistaken to be a new statement (in this example, “C + D” is a valid statement by itself if it’s not indented). This rule is also another throwback to the C language, where it is commonly used in “#define” macros; again, when in Pythonland, do as Pythonistas do, not as C programmers do.

Block rule special case

As mentioned previously, statements in a nested block of code are normally associated by being indented the same amount to the right. As one special case here, the body of a compound statement can instead appear on the same line as the header in Python, after the colon:

if x > y: print(x)

This allows us to code single-line if statements, single-line while and for loops, and so on. Here again, though, this will work only if the body of the compound statement itself does not contain any compound statements. That is, only simple statements—assignments, prints, function calls, and the like—are allowed after the colon. Larger statements must still appear on lines by themselves. Extra parts of compound statements (such as the else part of an if, which we’ll meet in the next section) must also be on separate lines of their own. Compound statement bodies can also consist of multiple simple statements separated by semicolons, but this tends to be frowned upon.

In general, even though it’s not always required, if you keep all your statements on individual lines and always indent your nested blocks, your code will be easier to read and change in the future. Moreover, some code profiling and coverage tools may not be able to distinguish between multiple statements squeezed onto a single line or the header and body of a one-line compound statement. It is almost always to your advantage to keep things simple in Python. You can use the special-case exceptions to write Python code that’s hard to read, but it takes a lot of work, and there are probably better ways to spend your time.

To see a prime and common exception to one of these rules in action, however (the use of a single-line if statement to break out of a loop), and to introduce more of Python’s syntax, let’s move on to the next section and write some real code.

A Quick Example: Interactive Loops

We’ll see all these syntax rules in action when we tour Python’s specific compound statements in the next few chapters, but they work the same everywhere in the Python language. To get started, let’s work through a brief, realistic example that demonstrates the way that statement syntax and statement nesting come together in practice, and introduces a few statements along the way.

A Simple Interactive Loop

Suppose you’re asked to write a Python program that interacts with a user in a console window. Maybe you’re accepting inputs to send to a database, or reading numbers to be used in a calculation. Regardless of the purpose, you need to code a loop that reads one or more inputs from a user typing on a keyboard, and prints back a result for each. In other words, you need to write a classic read/evaluate/print loop program.

In Python, typical boilerplate code for such an interactive loop might look like this:

while True:

    reply = input('Enter text:')

    if reply == 'stop': break

    print(reply.upper())

This code makes use of a few new ideas and some we’ve already seen:

§  The code leverages the Python while loop, Python’s most general looping statement. We’ll study the while statement in more detail later, but in short, it consists of the word while, followed by an expression that is interpreted as a true or false result, followed by a nested block of code that is repeated while the test at the top is true (the word True here is considered always true).

§  The input built-in function we met earlier in the book is used here for general console input—it prints its optional argument string as a prompt and returns the user’s typed reply as a string. Use raw_input in 2.X instead, per the upcoming note.

§  A single-line if statement that makes use of the special rule for nested blocks also appears here: the body of the if appears on the header line after the colon instead of being indented on a new line underneath it. This would work either way, but as it’s coded, we’ve saved an extra line.

§  Finally, the Python break statement is used to exit the loop immediately—it simply jumps out of the loop statement altogether, and the program continues after the loop. Without this exit statement, the while would loop forever, as its test is always true.

In effect, this combination of statements essentially means “read a line from the user and print it in uppercase until the user enters the word ‘stop.’” There are other ways to code such a loop, but the form used here is very common in Python code.

Notice that all three lines nested under the while header line are indented the same amount—because they line up vertically in a column this way, they are the block of code that is associated with the while test and repeated. Either the end of the source file or a lesser-indented statement will suffice to terminate the loop body block.

When this code is run, either interactively or as a script file, here is the sort of interaction we get—all of the code for this example is in interact.py in the book’s examples package:

Enter text:spam

SPAM

Enter text:42

42

Enter text:stop

NOTE

Version skew note: This example is coded for Python 3.X. If you are working in Python 2.X, the code works the same, but you must use raw_input instead of input in all of this chapter’s examples, and you can omit the outer parentheses in print statements (though they don’t hurt). In fact, if you study the interact.py file in the examples package, you’ll see that it does this automatically—to support 2.X compatibility, it resets input if the running Python’s major version is 2 (“input” winds up running raw_input):

import sys

if sys.version[0] == '2': input = raw_input   # 2.X compatible

In 3.X, raw_input was renamed input, and print is a built-in function instead of a statement (more on prints in the next chapter). Python 2.X has an input too, but it tries to evaluate the input string as though it were Python code, which probably won’t work in this context; eval(input()) can yield the same effect 3.X.

Doing Math on User Inputs

Our script works, but now suppose that instead of converting a text string to uppercase, we want to do some math with numeric input—squaring it, for example, perhaps in some misguided effort of an age-input program to tease its users. We might try statements like these to achieve the desired effect:

>>> reply = '20'

>>> reply ** 2

...error text omitted...

TypeError: unsupported operand type(s) for ** or pow(): 'str' and 'int'

This won’t quite work in our script, though, because (as discussed in the prior part of the book) Python won’t convert object types in expressions unless they are all numeric, and input from a user is always returned to our script as a string. We cannot raise a string of digits to a power unless we convert it manually to an integer:

>>> int(reply) ** 2

400

Armed with this information, we can now recode our loop to perform the necessary math. Type the following in a file to test it:

while True:

    reply = input('Enter text:')

    if reply == 'stop': break

    print(int(reply) ** 2)

print('Bye')

This script uses a single-line if statement to exit on “stop” as before, but it also converts inputs to perform the required math. This version also adds an exit message at the bottom. Because the print statement in the last line is not indented as much as the nested block of code, it is not considered part of the loop body and will run only once, after the loop is exited:

Enter text:2

4

Enter text:40

1600

Enter text:stop

Bye

NOTE

Usage note: From this point on I’ll assume that this code is stored in and run from a script file, via command line, IDLE menu option, or any of the other file launching techniques we met in Chapter 3. Again, it’s named interact.py in the book’s examples. If you are entering this code interactively, though, be sure to include a blank line (i.e., press Enter twice) before the final print statement, to terminate the loop. This implies that you also can’t cut and paste the code in its entirety into an interactive prompt: an extra blank line is required interactively, but not in script files. The final print doesn’t quite make sense in interactive mode, though—you’ll have to code it after interacting with the loop!

Handling Errors by Testing Inputs

So far so good, but notice what happens when the input is invalid:

Enter text:xxx

...error text omitted...

ValueError: invalid literal for int() with base 10: 'xxx'

The built-in int function raises an exception here in the face of a mistake. If we want our script to be robust, we can check the string’s content ahead of time with the string object’s isdigit method:

>>> S = '123'

>>> T = 'xxx'

>>> S.isdigit(), T.isdigit()

(True, False)

This also gives us an excuse to further nest the statements in our example. The following new version of our interactive script uses a full-blown if statement to work around the exception on errors:

while True:

    reply = input('Enter text:')

    if reply == 'stop':

        break

    elif not reply.isdigit():

        print('Bad!' * 8)

    else:

        print(int(reply) ** 2)

print('Bye')

We’ll study the if statement in more detail in Chapter 12, but it’s a fairly lightweight tool for coding logic in scripts. In its full form, it consists of the word if followed by a test and an associated block of code, one or more optional elif (“else if”) tests and code blocks, and an optionalelse part, with an associated block of code at the bottom to serve as a default. Python runs the block of code associated with the first test that is true, working from top to bottom, or the else part if all tests are false.

The if, elif, and else parts in the preceding example are associated as part of the same statement because they all line up vertically (i.e., share the same level of indentation). The if statement spans from the word if to the start of the print statement on the last line of the script. In turn, the entire if block is part of the while loop because all of it is indented under the loop’s header line. Statement nesting like this is natural once you get the hang of it.

When we run our new script, its code catches errors before they occur and prints an error message before continuing (which you’ll probably want to improve in a later release), but “stop” still gets us out, and valid numbers are still squared:

Enter text:5

25

Enter text:xyz

Bad!Bad!Bad!Bad!Bad!Bad!Bad!Bad!

Enter text:10

100

Enter text:stop

Handling Errors with try Statements

The preceding solution works, but as you’ll see later in the book, the most general way to handle errors in Python is to catch and recover from them completely using the Python try statement. We’ll explore this statement in depth in Part VII of this book, but as a preview, using a try here can lead to code that some would see as simpler than the prior version:

while True:

    reply = input('Enter text:')

    if reply == 'stop': break

    try:

        num = int(reply)

    except:

        print('Bad!' * 8)

    else:

        print(num ** 2)

print('Bye')

This version works exactly like the previous one, but we’ve replaced the explicit error check with code that assumes the conversion will work and wraps it in an exception handler for cases when it doesn’t. In other words, rather than detecting an error, we simply respond if one occurs.

This try statement is another compound statement, and follows the same pattern as if and while. It’s composed of the word try, followed by the main block of code (the action we are trying to run), followed by an except part that gives the exception handler code and an else part to be run if no exception is raised in the try part. Python first runs the try part, then runs either the except part (if an exception occurs) or the else part (if no exception occurs).

In terms of statement nesting, because the words try, except, and else are all indented to the same level, they are all considered part of the same single try statement. Notice that the else part is associated with the try here, not the if. As we’ve seen, else can appear in if statements in Python, but it can also appear in try statements and loops—its indentation tells you what statement it is a part of. In this case, the try statement spans from the word try through the code indented under the word else, because the else is indented the same as try. The if statement in this code is a one-liner and ends after the break.

Supporting floating-point numbers

Again, we’ll come back to the try statement later in this book. For now, be aware that because try can be used to intercept any error, it reduces the amount of error-checking code you have to write, and it’s a very general approach to dealing with unusual cases. If we’re sure that print won’t fail, for instance, this example could be even more concise:

while True:

    reply = input('Enter text:')

    if reply == 'stop': break

    try:

        print(int(reply) ** 2)

    except:

        print('Bad!' * 8)

print('Bye')

And if we wanted to support input of floating-point numbers instead of just integers, for example, using try would be much easier than manual error testing—we could simply run a float call and catch its exceptions:

while True:

    reply = input('Enter text:')

    if reply == 'stop': break

    try:

        print(float(reply) ** 2)

    except:

        print('Bad!' * 8)

print('Bye')

There is no isfloat for strings today, so this exception-based approach spares us from having to analyze all possible floating-point syntax in an explicit error check. When coding this way, we can enter a wider variety of numbers, but errors and exits still work as before:

Enter text:50

2500.0

Enter text:40.5

1640.25

Enter text:1.23E-100

1.5129e-200

Enter text:spam

Bad!Bad!Bad!Bad!Bad!Bad!Bad!Bad!

Enter text:stop

Bye

NOTE

Python’s eval call, which we used in Chapter 5 and Chapter 9 to convert data in strings and files, would work in place of float here too, and would allow input of arbitrary expressions (“2 ** 100” would be a legal, if curious, input, especially if we’re assuming the program is processing ages!). This is a powerful concept that is open to the same security issues mentioned in the prior chapters. If you can’t trust the source of a code string, use more restrictive conversion tools like int and float.

Python’s exec, used in Chapter 3 to run code read from a file, is similar to eval (but assumes the string is a statement instead of an expression and has no result), and its compile call precompiles frequently used code strings to bytecode objects for speed. Run a help on any of these for more details; as mentioned, exec is a statement in 2.X but a function in 3.X, so see its manual entry in 2.X instead. We’ll also use exec to import modules by name string in Chapter 25—an example of its more dynamic roles.

Nesting Code Three Levels Deep

Let’s look at one last mutation of our code. Nesting can take us even further if we need it to—we could, for example, extend our prior integer-only script to branch to one of a set of alternatives based on the relative magnitude of a valid input:

while True:

    reply = input('Enter text:')

    if reply == 'stop':

        break

    elif not reply.isdigit():

        print('Bad!' * 8)

    else:

        num = int(reply)

        if num < 20:

            print('low')

        else:

            print(num ** 2)

print('Bye')

This version adds an if statement nested in the else clause of another if statement, which is in turn nested in the while loop. When code is conditional or repeated like this, we simply indent it further to the right. The net effect is like that of prior versions, but we’ll now print “low” for numbers less than 20:

Enter text:19

low

Enter text:20

400

Enter text:spam

Bad!Bad!Bad!Bad!Bad!Bad!Bad!Bad!

Enter text:stop

Bye

Chapter Summary

That concludes our quick look at Python statement syntax. This chapter introduced the general rules for coding statements and blocks of code. As you’ve learned, in Python we normally code one statement per line and indent all the statements in a nested block the same amount (indentation is part of Python’s syntax). However, we also looked at a few exceptions to these rules, including continuation lines and single-line tests and loops. Finally, we put these ideas to work in an interactive script that demonstrated a handful of statements and showed statement syntax in action.

In the next chapter, we’ll start to dig deeper by going over each of Python’s basic procedural statements in depth. As you’ll see, though, all statements follow the same general rules introduced here.

Test Your Knowledge: Quiz

1.    What three things are required in a C-like language but omitted in Python?

2.    How is a statement normally terminated in Python?

3.    How are the statements in a nested block of code normally associated in Python?

4.    How can you make a single statement span multiple lines?

5.    How can you code a compound statement on a single line?

6.    Is there any valid reason to type a semicolon at the end of a statement in Python?

7.    What is a try statement for?

8.    What is the most common coding mistake among Python beginners?

Test Your Knowledge: Answers

1.    C-like languages require parentheses around the tests in some statements, semicolons at the end of each statement, and braces around a nested block of code.

2.    The end of a line terminates the statement that appears on that line. Alternatively, if more than one statement appears on the same line, they can be terminated with semicolons; similarly, if a statement spans many lines, you must terminate it by closing a bracketed syntactic pair.

3.    The statements in a nested block are all indented the same number of tabs or spaces.

4.    You can make a statement span many lines by enclosing part of it in parentheses, square brackets, or curly braces; the statement ends when Python sees a line that contains the closing part of the pair.

5.    The body of a compound statement can be moved to the header line after the colon, but only if the body consists of only noncompound statements.

6.    Only when you need to squeeze more than one statement onto a single line of code. Even then, this only works if all the statements are noncompound, and it’s discouraged because it can lead to code that is difficult to read.

7.    The try statement is used to catch and recover from exceptions (errors) in a Python script. It’s usually an alternative to manually checking for errors in your code.

8.    Forgetting to type the colon character at the end of the header line in a compound statement is the most common beginner’s mistake. If you’re new to Python and haven’t made it yet, you probably will soon!