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<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"

"http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<head>

<title>Files — Pense Python 2e documentation</title>

var DOCUMENTATION_OPTIONS = {

URL_ROOT: './',

VERSION: '2e',

COLLAPSE_INDEX: false,

FILE_SUFFIX: '.html',

HAS_SOURCE: true

};

</script>

</head>

<h1>Files<a class="headerlink" href="#files" title="Permalink to this headline">¶</a></h1>

This chapter introduces the idea of “persistent” programs that keep data

in permanent storage, and shows how to use different kinds of permanent

storage, like files and databases.

<h2>Persistence<a class="headerlink" href="#persistence" title="Permalink to this headline">¶</a></h2>

Most of the programs we have seen so far are transient in the sense that

they run for a short time and produce some output, but when they end,

their data disappears. If you run the program again, it starts with a

clean slate.

Other programs are persistent: they run for a long time (or all the

time); they keep at least some of their data in permanent storage (a

hard drive, for example); and if they shut down and restart, they pick

up where they left off.

Examples of persistent programs are operating systems, which run pretty

much whenever a computer is on, and web servers, which run all the time,

waiting for requests to come in on the network.

One of the simplest ways for programs to maintain their data is by

reading and writing text files. We have already seen programs that read

text files; in this chapter we will see programs that write them.

An alternative is to store the state of the program in a database. In

this chapter I will present a simple database and a module, pickle, that

makes it easy to store program data.

</div>

<h2>Reading and writing<a class="headerlink" href="#reading-and-writing" title="Permalink to this headline">¶</a></h2>

A text file is a sequence of characters stored on a permanent medium

like a hard drive, flash memory, or CD-ROM. We saw how to open and read

a file in Section [wordlist].

To write a file, you have to open it with mode <code class="docutils literal">'w'</code> as a second

parameter:

<div class="highlight-python"><div class="highlight"><pre>>>> fout = open('output.txt', 'w')

</pre></div>

</div>

If the file already exists, opening it in write mode clears out the old

data and starts fresh, so be careful! If the file doesn’t exist, a new

one is created.

open returns a file object that provides methods for working with the

file. The write method puts data into the file.

<div class="highlight-python"><div class="highlight"><pre>>>> line1 = "This here's the wattle,\n"

>>> fout.write(line1)

24

</pre></div>

</div>

The return value is the number of characters that were written. The file

object keeps track of where it is, so if you call write again, it adds

the new data to the end of the file.

<div class="highlight-python"><div class="highlight"><pre>>>> line2 = "the emblem of our land.\n"

>>> fout.write(line2)

24

</pre></div>

</div>

When you are done writing, you should close the file.

<div class="highlight-python"><div class="highlight"><pre>>>> fout.close()

</pre></div>

</div>

If you don’t close the file, it gets closed for you when the program

ends.

</div>

<h2>Format operator<a class="headerlink" href="#format-operator" title="Permalink to this headline">¶</a></h2>

The argument of write has to be a string, so if we want to put other

values in a file, we have to convert them to strings. The easiest way to

do that is with str:

<div class="highlight-python"><div class="highlight"><pre>>>> x = 52

>>> fout.write(str(x))

</pre></div>

</div>

An alternative is to use the format operator, %. When applied to

integers, % is the modulus operator. But when the first operand is a

string, % is the format operator.

The first operand is the format string, which contains one or more

format sequences, which specify how the second operand is formatted.

The result is a string.

For example, the format sequence <code class="docutils literal">'%d'</code> means that the second operand

should be formatted as a decimal integer:

<div class="highlight-python"><div class="highlight"><pre>>>> camels = 42

>>> '%d' % camels

'42'

</pre></div>

</div>

The result is the string <code class="docutils literal">'42'</code>, which is not to be confused with the

integer value 42.

A format sequence can appear anywhere in the string, so you can embed a

value in a sentence:

<div class="highlight-python"><div class="highlight"><pre>>>> 'I have spotted %d camels.' % camels

'I have spotted 42 camels.'

</pre></div>

</div>

If there is more than one format sequence in the string, the second

argument has to be a tuple. Each format sequence is matched with an

element of the tuple, in order.

The following example uses <code class="docutils literal">'%d'</code> to format an integer, <code class="docutils literal">'%g'</code> to

format a floating-point number, and <code class="docutils literal">'%s'</code> to format a string:

<div class="highlight-python"><div class="highlight"><pre>>>> 'In %d years I have spotted %g %s.' % (3, 0.1, 'camels')

'In 3 years I have spotted 0.1 camels.'

</pre></div>

</div>

The number of elements in the tuple has to match the number of format

sequences in the string. Also, the types of the elements have to match

the format sequences:

<div class="highlight-python"><div class="highlight"><pre>>>> '%d %d %d' % (1, 2)

TypeError: not enough arguments for format string

>>> '%d' % 'dollars'

TypeError: %d format: a number is required, not str

</pre></div>

</div>

In the first example, there aren’t enough elements; in the second, the

element is the wrong type.

For more information on the format operator, see

<a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting">https://docs.python.org/3/library/stdtypes.html#printf-style-string-formatting</a>.

A more powerful alternative is the string format method, which you can

read about at

<a class="reference external" href="https://docs.python.org/3/library/stdtypes.html#str.format">https://docs.python.org/3/library/stdtypes.html#str.format</a>.

</div>

<h2>Filenames and paths<a class="headerlink" href="#filenames-and-paths" title="Permalink to this headline">¶</a></h2>

Files are organized into directories (also called “folders”). Every

running program has a “current directory”, which is the default

directory for most operations. For example, when you open a file for

reading, Python looks for it in the current directory.

The os module provides functions for working with files and directories

(“os” stands for “operating system”). os.getcwd returns the name of the

current directory:

<div class="highlight-python"><div class="highlight"><pre>>>> import os

>>> cwd = os.getcwd()

>>> cwd

'/home/dinsdale'

</pre></div>

</div>

cwd stands for “current working directory”. The result in this example

is /home/dinsdale, which is the home directory of a user named dinsdale.

A string like <code class="docutils literal">'/home/dinsdale'</code> that identifies a file or directory

is called a path.

A simple filename, like memo.txt is also considered a path, but it is a

relative path because it relates to the current directory. If the

current directory is /home/dinsdale, the filename memo.txt would refer

to /home/dinsdale/memo.txt.

A path that begins with / does not depend on the current directory; it

is called an absolute path. To find the absolute path to a file, you

can use os.path.abspath:

<div class="highlight-python"><div class="highlight"><pre>>>> os.path.abspath('memo.txt')

'/home/dinsdale/memo.txt'

</pre></div>

</div>

os.path provides other functions for working with filenames and paths.

For example, os.path.exists checks whether a file or directory exists:

<div class="highlight-python"><div class="highlight"><pre>>>> os.path.exists('memo.txt')

True

</pre></div>

</div>

If it exists, os.path.isdir checks whether it’s a directory:

<div class="highlight-python"><div class="highlight"><pre>>>> os.path.isdir('memo.txt')

False

>>> os.path.isdir('/home/dinsdale')

True

</pre></div>

</div>

Similarly, os.path.isfile checks whether it’s a file.

os.listdir returns a list of the files (and other directories) in the

given directory:

<div class="highlight-python"><div class="highlight"><pre>>>> os.listdir(cwd)

['music', 'photos', 'memo.txt']

</pre></div>

</div>

To demonstrate these functions, the following example “walks” through a

directory, prints the names of all the files, and calls itself

recursively on all the directories.

<div class="highlight-python"><div class="highlight"><pre>def walk(dirname):

for name in os.listdir(dirname):

path = os.path.join(dirname, name)

if os.path.isfile(path):

print(path)

else:

walk(path)

</pre></div>

</div>

os.path.join takes a directory and a file name and joins them into a

complete path.

The os module provides a function called walk that is similar to this

one but more versatile. As an exercise, read the documentation and use

it to print the names of the files in a given directory and its

subdirectories. You can download my solution from

<a class="reference external" href="http://thinkpython2.com/code/walk.py">http://thinkpython2.com/code/walk.py</a>.

</div>

<h2>Catching exceptions<a class="headerlink" href="#catching-exceptions" title="Permalink to this headline">¶</a></h2>

A lot of things can go wrong when you try to read and write files. If

you try to open a file that doesn’t exist, you get an IOError:

<div class="highlight-python"><div class="highlight"><pre>>>> fin = open('bad_file')

IOError: [Errno 2] No such file or directory: 'bad_file'

</pre></div>

</div>

If you don’t have permission to access a file:

<div class="highlight-python"><div class="highlight"><pre>>>> fout = open('/etc/passwd', 'w')

PermissionError: [Errno 13] Permission denied: '/etc/passwd'

</pre></div>

</div>

And if you try to open a directory for reading, you get

IsADirectoryError: [Errno 21] Is a directory: '/home'

</pre></div>

</div>

To avoid these errors, you could use functions like os.path.exists and

os.path.isfile, but it would take a lot of time and code to check all

the possibilities (if “Errno 21” is any indication, there are at least

21 things that can go wrong).

It is better to go ahead and try—and deal with problems if they

happen—which is exactly what the try statement does. The syntax is

similar to an if...else statement:

<div class="highlight-python"><div class="highlight"><pre>try:

fin = open('bad_file')

except:

print('Something went wrong.')

</pre></div>

</div>

Python starts by executing the try clause. If all goes well, it skips

the except clause and proceeds. If an exception occurs, it jumps out of

the try clause and runs the except clause.

Handling an exception with a try statement is called catching an

exception. In this example, the except clause prints an error message

that is not very helpful. In general, catching an exception gives you a

chance to fix the problem, or try again, or at least end the program

gracefully.

</div>

<h2>Databases<a class="headerlink" href="#databases" title="Permalink to this headline">¶</a></h2>

A database is a file that is organized for storing data. Many

databases are organized like a dictionary in the sense that they map

from keys to values. The biggest difference between a database and a

dictionary is that the database is on disk (or other permanent storage),

so it persists after the program ends.

The module dbm provides an interface for creating and updating database

files. As an example, I’ll create a database that contains captions for

image files.

Opening a database is similar to opening other files:

<div class="highlight-python"><div class="highlight"><pre>>>> import dbm

>>> db = dbm.open('captions', 'c')

</pre></div>

</div>

The mode <code class="docutils literal">'c'</code> means that the database should be created if it doesn’t

already exist. The result is a database object that can be used (for

most operations) like a dictionary.

When you create a new item, dbm updates the database file.

<div class="highlight-python"><div class="highlight"><pre>>>> db['cleese.png'] = 'Photo of John Cleese.'

</pre></div>

</div>

When you access one of the items, dbm reads the file:

<div class="highlight-python"><div class="highlight"><pre>>>> db['cleese.png']

b'Photo of John Cleese.'

</pre></div>

</div>

The result is a bytes object, which is why it begins with b. A bytes

object is similar to a string in many ways. When you get farther into

Python, the difference becomes important, but for now we can ignore it.

If you make another assignment to an existing key, dbm replaces the old

value:

>>> db['cleese.png']

b'Photo of John Cleese doing a silly walk.'

</pre></div>

</div>

Some dictionary methods, like keys and items, don’t work with database

objects. But iteration with a for loop works:

<div class="highlight-python"><div class="highlight"><pre>for key in db:

print(key, db[key])

</pre></div>

</div>

As with other files, you should close the database when you are done:

<div class="highlight-python"><div class="highlight"><pre>>>> db.close()

</pre></div>

</div>

<h2>Pickling<a class="headerlink" href="#pickling" title="Permalink to this headline">¶</a></h2>

A limitation of dbm is that the keys and values have to be strings or

bytes. If you try to use any other type, you get an error.

The pickle module can help. It translates almost any type of object into

a string suitable for storage in a database, and then translates strings

back into objects.

pickle.dumps takes an object as a parameter and returns a string

representation (dumps is short for “dump string”):

<div class="highlight-python"><div class="highlight"><pre>>>> import pickle

>>> t = [1, 2, 3]

>>> pickle.dumps(t)

b'\x80\x03]q\x00(K\x01K\x02K\x03e.'

</pre></div>

</div>

The format isn’t obvious to human readers; it is meant to be easy for

pickle to interpret. pickle.loads (“load string”) reconstitutes the

object:

<div class="highlight-python"><div class="highlight"><pre>>>> t1 = [1, 2, 3]

>>> s = pickle.dumps(t1)

>>> t2 = pickle.loads(s)

>>> t2

[1, 2, 3]

</pre></div>

</div>

Although the new object has the same value as the old, it is not (in

general) the same object:

<div class="highlight-python"><div class="highlight"><pre>>>> t1 == t2

True

>>> t1 is t2

False

</pre></div>

</div>

In other words, pickling and then unpickling has the same effect as

copying the object.

You can use pickle to store non-strings in a database. In fact, this

combination is so common that it has been encapsulated in a module

called shelve.

</div>

<h2>Pipes<a class="headerlink" href="#pipes" title="Permalink to this headline">¶</a></h2>

Most operating systems provide a command-line interface, also known as a

shell. Shells usually provide commands to navigate the file system

and launch applications. For example, in Unix you can change directories

with cd, display the contents of a directory with ls, and launch a web

browser by typing (for example) firefox.

Any program that you can launch from the shell can also be launched from

Python using a pipe object, which represents a running program.

For example, the Unix command ls -l normally displays the contents of

the current directory in long format. You can launch ls with

os.popen <a href="#id2">[1]_</a>:

<div class="highlight-python"><div class="highlight"><pre>>>> cmd = 'ls -l'

>>> fp = os.popen(cmd)

</pre></div>

</div>

The argument is a string that contains a shell command. The return value

is an object that behaves like an open file. You can read the output

from the ls process one line at a time with readline or get the whole

thing at once with read:

<div class="highlight-python"><div class="highlight"><pre>>>> res = fp.read()

</pre></div>

</div>

When you are done, you close the pipe like a file:

<div class="highlight-python"><div class="highlight"><pre>>>> stat = fp.close()

>>> print(stat)

None

</pre></div>

</div>

The return value is the final status of the ls process; None means that

it ended normally (with no errors).

For example, most Unix systems provide a command called md5sum that

reads the contents of a file and computes a “checksum”. You can read

about MD5 at <a class="reference external" href="http://en.wikipedia.org/wiki/Md5">http://en.wikipedia.org/wiki/Md5</a>. This command provides an

efficient way to check whether two files have the same contents. The

probability that different contents yield the same checksum is very

small (that is, unlikely to happen before the universe collapses).

You can use a pipe to run md5sum from Python and get the result:

<div class="highlight-python"><div class="highlight"><pre>>>> filename = 'book.tex'

>>> cmd = 'md5sum ' + filename

>>> res = fp.read()

>>> stat = fp.close()

>>> print(res)

1e0033f0ed0656636de0d75144ba32e0 book.tex

>>> print(stat)

None

</pre></div>

</div>

<h2>Writing modules<a class="headerlink" href="#writing-modules" title="Permalink to this headline">¶</a></h2>

Any file that contains Python code can be imported as a module. For

example, suppose you have a file named wc.py with the following code:

<div class="highlight-python"><div class="highlight"><pre>def linecount(filename):

count = 0

for line in open(filename):

count += 1

return count

print(linecount('wc.py'))

</pre></div>

</div>

If you run this program, it reads itself and prints the number of lines

in the file, which is 7. You can also import it like this:

<div class="highlight-python"><div class="highlight"><pre>>>> import wc

7

</pre></div>

</div>

Now you have a module object wc:

<div class="highlight-python"><div class="highlight"><pre>>>> wc

</pre></div>

</div>

The module object provides <code class="docutils literal">linecount</code>:

<div class="highlight-python"><div class="highlight"><pre>>>> wc.linecount('wc.py')

7

</pre></div>

</div>

So that’s how you write modules in Python.

The only problem with this example is that when you import the module it

runs the test code at the bottom. Normally when you import a module, it

defines new functions but it doesn’t run them.

Programs that will be imported as modules often use the following idiom:

<div class="highlight-python"><div class="highlight"><pre>if __name__ == '__main__':

print(linecount('wc.py'))

</pre></div>

</div>

<code class="docutils literal">__name__</code> is a built-in variable that is set when the program starts.

If the program is running as a script, <code class="docutils literal">__name__</code> has the value

<code class="docutils literal">'__main__'</code>; in that case, the test code runs. Otherwise, if the

module is being imported, the test code is skipped.

As an exercise, type this example into a file named wc.py and run it as

a script. Then run the Python interpreter and import wc. What is the

value of <code class="docutils literal">__name__</code> when the module is being imported?

Warning: If you import a module that has already been imported, Python

does nothing. It does not re-read the file, even if it has changed.

If you want to reload a module, you can use the built-in function

reload, but it can be tricky, so the safest thing to do is restart the

interpreter and then import the module again.

</div>

<h2>Debugging<a class="headerlink" href="#debugging" title="Permalink to this headline">¶</a></h2>

When you are reading and writing files, you might run into problems with

whitespace. These errors can be hard to debug because spaces, tabs and

newlines are normally invisible:

<div class="highlight-python"><div class="highlight"><pre>>>> s = '1 2\t 3\n 4'

>>> print(s)

1 2 3

4

</pre></div>

</div>

The built-in function repr can help. It takes any object as an argument

and returns a string representation of the object. For strings, it

represents whitespace characters with backslash sequences:

<div class="highlight-python"><div class="highlight"><pre>>>> print(repr(s))

'1 2\t 3\n 4'

</pre></div>

</div>

This can be helpful for debugging.

One other problem you might run into is that different systems use

different characters to indicate the end of a line. Some systems use a

newline, represented <code class="docutils literal">\n</code>. Others use a return character, represented

<code class="docutils literal">\r</code>. Some use both. If you move files between different systems,

these inconsistencies can cause problems.

For most systems, there are applications to convert from one format to

another. You can find them (and read more about this issue) at

<a class="reference external" href="http://en.wikipedia.org/wiki/Newline">http://en.wikipedia.org/wiki/Newline</a>. Or, of course, you could write one

yourself.

</div>

<h2>Glossary<a class="headerlink" href="#glossary" title="Permalink to this headline">¶</a></h2>

<dt>persistente (persistent)</dt>

<dd>Pertaining to a program that runs indefinitely and keeps at least some of its data in permanent storage.</dd>

<dt>operador de formatação (format operator)</dt>

<dd>An operator, %, that takes a format string and a tuple and generates a string that includes the elements of the tuple formatted as specified by the format string.</dd>

<dt>string de formatação (format string)</dt>

<dd>A string, used with the format operator, that contains format sequences.</dd>

<dt>sequência de formatação (format sequence)</dt>

<dd>A sequence of characters in a format string, like %d, that specifies how a value should be formatted.</dd>

<dt>arquivo-texto (text file)</dt>

<dd>A sequence of characters stored in permanent storage like a hard drive.</dd>

<dt>diretório (directory)</dt>

<dd>A named collection of files, also called a folder.</dd>

<dt>caminho (path)</dt>

<dd>A string that identifies a file.</dd>

<dt>caminho relativo (relative path)</dt>

<dd>A path that starts from the current directory.</dd>

<dt>caminho absoluto (absolute path)</dt>

<dd>A path that starts from the topmost directory in the file system.</dd>

<dt>capturar (catch)</dt>

<dd>To prevent an exception from terminating a program using the try and except statements.</dd>

<dt>banco de dados (database)</dt>

<dd>A file whose contents are organized like a dictionary with keys that correspond to values.</dd>

<dt>objeto <code class="docutils literal">bytes</code> (<code class="docutils literal">bytes</code> object)</dt>

<dd>An object similar to a string.</dd>

<dt>shell</dt>

<dd>A program that allows users to type commands and then executes them by starting other programs.</dd>

<dt>objeto “pipe” (pipe object)</dt>

<dd>An object that represents a running program, allowing a Python program to run commands and read the results.</dd>

</dl>

</div>

<h2>Exercises<a class="headerlink" href="#exercises" title="Permalink to this headline">¶</a></h2>

Write a function called sed that takes as arguments a pattern string, a

replacement string, and two filenames; it should read the first file and

write the contents into the second file (creating it if necessary). If

the pattern string appears anywhere in the file, it should be replaced

with the replacement string.

If an error occurs while opening, reading, writing or closing files,

your program should catch the exception, print an error message, and

exit. Solution: <a class="reference external" href="http://thinkpython2.com/code/sed.py">http://thinkpython2.com/code/sed.py</a>.

If you download my solution to Exercise [anagrams] from

<a class="reference external" href="http://thinkpython2.com/code/anagram_sets.py">http://thinkpython2.com/code/anagram_sets.py</a>, you’ll see that it creates

a dictionary that maps from a sorted string of letters to the list of

words that can be spelled with those letters. For example, <code class="docutils literal">'opst'</code>

maps to the list <code class="docutils literal">['opts', 'post', 'pots', 'spot', 'stop', 'tops']</code>.

Write a module that imports <code class="docutils literal">anagram_sets</code> and provides two new

functions: <code class="docutils literal">store_anagrams</code> should store the anagram dictionary in a

“shelf”; <code class="docutils literal">read_anagrams</code> should look up a word and return a list of

its anagrams. Solution: <a class="reference external" href="http://thinkpython2.com/code/anagram_db.py">http://thinkpython2.com/code/anagram_db.py</a>.

[checksum]

In a large collection of MP3 files, there may be more than one copy of

the same song, stored in different directories or with different file

names. The goal of this exercise is to search for duplicates.

<li>Write a program that searches a directory and all of its

subdirectories, recursively, and returns a list of complete paths for

all files with a given suffix (like .mp3). Hint: os.path provides

several useful functions for manipulating file and path names.</li>

<li>To recognize duplicates, you can use md5sum to compute a “checksum”

for each files. If two files have the same checksum, they probably

have the same contents.</li>

<li>To double-check, you can use the Unix command diff.</li>

</ol>

Solution: <a class="reference external" href="http://thinkpython2.com/code/find_duplicates.py">http://thinkpython2.com/code/find_duplicates.py</a>.

</div>

<h3><a href="index.html">Table Of Contents</a></h3>

<ul>

<li><a class="reference internal" href="#">Files</a><ul>

<li><a class="reference internal" href="#persistence">Persistence</a></li>

<li><a class="reference internal" href="#reading-and-writing">Reading and writing</a></li>

<li><a class="reference internal" href="#format-operator">Format operator</a></li>

<li><a class="reference internal" href="#filenames-and-paths">Filenames and paths</a></li>

<li><a class="reference internal" href="#catching-exceptions">Catching exceptions</a></li>

<li><a class="reference internal" href="#databases">Databases</a></li>

<li><a class="reference internal" href="#pickling">Pickling</a></li>

<li><a class="reference internal" href="#pipes">Pipes</a></li>

<li><a class="reference internal" href="#writing-modules">Writing modules</a></li>

<li><a class="reference internal" href="#debugging">Debugging</a></li>

<li><a class="reference internal" href="#glossary">Glossary</a></li>

<li><a class="reference internal" href="#exercises">Exercises</a></li>

</ul>

</li>

</ul>

<h3>Related Topics</h3>

<ul>

<li><a href="index.html">Documentation overview</a><ul>

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<li>Next: <a href="15-cls-object.html" title="next chapter">Classes and objects</a></li>

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