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>>> for i in range(3):

...

print('PapayaWhip')

①

PapayaWhip

>>> import sys

>>> for i in range(3):

... sys.stdout.write('is the')

②

is theis theis the

>>> for i in range(3):

... sys.stderr.write('new black')

③

new blacknew blacknew black

1. The

print()

function, in aloop. Nothing surprising here.

stdout

is defined in the

sys

module, and it is a

stream object. Calling its

write()

function will print out

whatever string you give it. In fact, this is what the

function really does; it adds a carriagereturn to

the end of the string you’reprinting, and calls

sys.stdout.write

3. In the simplest case,

sys.stdout

and

sys.stderr

send their output to thesameplace: the Python

IDE

(if

you’rein one), or theterminal (if you’re running Python from the command line). Likestandard output,

standard error does not add carriage returns for you. If you want carriagereturns, you’ll need to write

carriagereturn characters.

sys.stdout

and

sys.stderr

are stream objects, but they are write-only. Attempting to call their

read()

method will always raise an

IOError

>>> import sys

>>> sys.stdout.read()

Traceback (most recent call last):

File "<stdin>", line 1, in <module>

IOError: not readable

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11.6.1. R

EDIRECTING

TANDARD

UTPUT

sys.stdout

and

sys.stderr

are stream objects, albeit ones that only support writing. But they’re not

constants;they’re variables. That means you can assign them a new value—any other stream object—to

redirect their output.

import sys

class RedirectStdoutTo:

def __init__(self, out_new):

self.out_new = out_new

def __enter__(self):

self.out_old = sys.stdout

sys.stdout = self.out_new

def __exit__(self, *args):

sys.stdout = self.out_old

print('A')

with open('out.log', mode='w', encoding='utf-8') as a_file, RedirectStdoutTo(a_file):

print('B')

print('C')

Check this out:

you@localhost:~/diveintopython3/examples$ python3 stdout.py

you@localhost:~/diveintopython3/examples$ cat out.log

Did you get this error?

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you@localhost:~/diveintopython3/examples$ python3 stdout.py

File "stdout.py", line 15

with open('out.log', mode='w', encoding='utf-8') as a_file, RedirectStdoutTo(a_file):

SyntaxError: invalid syntax

If so, you’re probably using Python 3.0. You should really upgradeto Python 3.1.

Python 3.0 supported the

with

statement, but each statement can only use onecontext

manager. Python 3.1 allows you to chain multiple context managers in a single

with

statement.

Let’s take the last part first.

print('A')

with open('out.log', mode='w', encoding='utf-8') as a_file, RedirectStdoutTo(a_file):

print('B')

print('C')

That’s acomplicated

with

statement. Let me rewriteit as something morerecognizable.

with open('out.log', mode='w', encoding='utf-8') as a_file:

with RedirectStdoutTo(a_file):

print('B')

As the rewriteshows, you actually have two

with

statements, one nested within the scopeof theother. The

“outer”

with

statement should befamiliar by now: it opens a

UTF

-8-encoded text filenamed

out.log

for

writing and assigns the stream object to a variable named

a_file

.But that’s not the only thing odd here.

with RedirectStdoutTo(a_file):

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Where’s the

clause? The

with

statement doesn’t actually require one. Just like you can call a function and

ignore its return value, you can have a

with

statement that doesn’t assign the

with

context to a variable. In

this case, you’re only interested in theside effects of the

RedirectStdoutTo

context.

What are those sideeffects? Takea look inside the

RedirectStdoutTo

class. This class is acustom

context

manager. Any class can bea context manager by defining two

special methods:

__enter__()

and

__exit__()

class RedirectStdoutTo:

def __init__(self, out_new):

①

self.out_new = out_new

def __enter__(self):

②

self.out_old = sys.stdout

sys.stdout = self.out_new

def __exit__(self, *args):

③

sys.stdout = self.out_old

1. The

__init__()

method is called immediately after an instance is created. It takes one parameter, the stream

object that you want to use as standard output for the life of the context. This method just saves the

stream object in an instance variable so other methods can useit later.

2. The

__enter__()

method is a

special class method;Python calls it when entering a context (i.e. at the

beginning of the

with

statement). This method saves the current valueof

sys.stdout

self.out_old

,then

redirects standard output by assigning

self.out_new

sys.stdout

3. The

__exit__()

method is another special class method;Python calls it when exiting the context (i.e. at the

end of the

with

statement). This method restores standard output to its original value by assigning the saved

self.out_old

value to

sys.stdout

Putting it all together:

print('A')

①

with open('out.log', mode='w', encoding='utf-8') as a_file, RedirectStdoutTo(a_file):

②

print('B')

③

print('C')

④

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1. This will print to the

IDE

“InteractiveWindow” (or theterminal, if running the script from the command

line).

2. This

with

statement takes a comma-separated list of contexts. The comma-separated list acts like aseries of

nested

with

blocks. Thefirst context listed is the “outer” block; the last one listed is the“inner”block. The

first context opens a file; the second context redirects

sys.stdout

tothestream object that was created in

the first context.

3. Becausethis

print()

function is executed with the context created by the

with

statement, it will not print

to thescreen; it will write to thefile

out.log

4. The

with

codeblockis over. Python has told each context manager to do whatever it is they do upon

exiting a context. The context managers form alast-in-first-out stack. Upon exiting, the second context

changed

sys.stdout

backto its original value, then the first context closed the filenamed

out.log

.Since

standard output has been restored toits original value, calling the

print()

function will onceagain print to

the screen.

Redirecting standard error works exactly the same way, using

sys.stderr

instead of

sys.stdout

⁂

11.7.F

URTHER

EADING

•

Reading and writing files in thePython.org tutorial

•

module

•

Stream objects

•

Context manager types

•

sys.stdout

and

sys.stderr

•

FUSE

on Wikipedia

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HAPTER

12. XML

❝

In the archonship of Aristaechmus, Draco enacted hisordinances.

❞

—

Aristotle

12.1. D

IVING

ost of the chapters in this bookhave centered around a piece of samplecode. But

XML

isn’t about

code;it’s about data. One common use of

XML

is “syndication feeds” that list thelatest articles on a blog,

forum, or other frequently-updated website. Most popular bloggingsoftware can produce a feed and update

it whenever new articles, discussion threads, or blog posts are published. You can follow a blogby

“subscribing” toits feed, and you can follow multipleblogs with a dedicated “

feed aggregator”like

Google

Reader.

Here, then, is the

XML

data we’ll be working with in this chapter. It’s a feed—specifically, an

Atom

syndication feed.

286

<?xml version='1.0' encoding='utf-8'?>

<subtitle>currently between addictions</subtitle>

<id>tag:diveintomark.org,2001-07-29:/</id>

<entry>

<uri>http://diveintomark.org/</uri>

</author>

<title>Dive into history, 2009 edition</title>

href='http://diveintomark.org/archives/2009/03/27/dive-into-history-2009-edition'/>

<id>tag:diveintomark.org,2009-03-27:/archives/20090327172042</id>

<summary type='html'>Putting an entire chapter on one page sounds

bloated, but consider this &mdash; my longest chapter so far

would be 75 printed pages, and it loads in under 5 seconds&hellip;

On dialup.</summary>

</entry>

<entry>

<uri>http://diveintomark.org/</uri>

</author>

<title>Accessibility is a harsh mistress</title>

287

href='http://diveintomark.org/archives/2009/03/21/accessibility-is-a-harsh-mistress'/>

<id>tag:diveintomark.org,2009-03-21:/archives/20090321200928</id>

<summary type='html'>The accessibility orthodoxy does not permit people to

question the value of features that are rarely useful and rarely used.</summary>

</entry>

<entry>

</author>

<title>A gentle introduction to video encoding, part 1: container formats</title>

href='http://diveintomark.org/archives/2008/12/18/give-part-1-container-formats'/>

<id>tag:diveintomark.org,2008-12-18:/archives/20081218155422</id>

<summary type='html'>These notes will eventually become part of a

tech talk on video encoding.</summary>

</entry>

</feed>

⁂

288

12.2.A 5-M

INUTE

RASH

OURSEIN

XML

If you already know about

XML

,you can skip this section.

XML

is a generalized way of describing hierarchical structured data. An

XML

document contains one or more

elements, which are delimited by start and end tags. This is a complete (albeit boring)

XML

document:

<foo>

①

</foo>

②

1. This is the start tag of the

foo

element.

2. This is the matching end tag of the

foo

element. Likebalancing parentheses in writing or mathematics or

code, every start tag must be closed (matched) by a corresponding end tag.

Elements can benested to any depth. An element

bar

insidean element

foo

is said to be a subelement or

child of

foo

<foo>

</foo>

Thefirst element in every

XML

document is called theroot element. An

XML

document can only have one

root element. Thefollowing is not an

XML

document, becauseit has two root elements:

Elements can have attributes, which arename-valuepairs. Attributes are listed within thestart tag of an

element and separated by whitespace. Attribute names can not be repeated within an element. Attribute values

must be quoted. You may use either single or doublequotes.

<foo

lang='en'>

①

lang="fr"></bar>

②

</foo>

289

1. The

foo

element has one attribute, named

lang

.Thevalueof its

lang

attribute is

2. The

bar

element has two attributes, named

and

lang

.Thevalueof its

lang

attribute is

.This doesn’t

conflict with the

foo

element in any way. Each element has its own set of attributes.

If an element has morethan one attribute, the ordering of the attributes is not significant. An element’s

attributes form an unordered set of keys and values, like a Python dictionary. There is no limit to the

number of attributes you can define on each element.

Elements can have text content.

PapayaWhip</bar>

</foo>

Elements that contain no text and no children are empty.

Thereis a shorthand for writing empty elements. By putting a

character in thestart tag, you can skip the

end tag altogther. The

XML

document in the previous example could bewritten like this instead:

<foo

Like Python functions can be declared in different modules,

XML

elements can be declared in different

namespaces. Namespaces usually looklike URLs. You use an

xmlns

declaration to define adefault namespace.

Anamespace declaration looks similar to an attribute, but it has a different purpose.

<feed

xmlns='http://www.w3.org/2005/Atom'>

①

②

</feed>

1. The

feed

element is in the

http://www.w3.org/2005/Atom

namespace.

2. The

title

element is also in the

http://www.w3.org/2005/Atom

namespace. Thenamespace declaration affects

the element whereit’s declared, plus all child elements.

290

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