Python advanced 3.the python std lib by example – system related modules

THE PYTHON STD LIB BY EXAMPLE
– PART 4: DATE,TIME AND SYSTEM
RELATED MODULES
John
Saturday, December 21, 2013

Brief introduction
• The time module includes clock time and the
processor run-time
• The datetime module provide a higher-level
interface for date, time and combined values.
It support arithmetic,comparison, and time
zone configuration.
• The calendar module includeweeks,months,
and years.

Function time – clock time
• Function time return the number of seconds
since the start of epoch
• Function ctime show human-readable
format.

Function clock – processor clock
time
• Use it for perfomance testing, beachmarking.
• function time.clock()
>>>import time
>>>for i in range(6,1,-1):
print '%s %0.2f %0.2f' % (time.ctime(),time.time(),time.clock())
print 'sleeping', i
time.sleep(i)

Datetime module: doing time and
date parsing
• Class datetime.time: has attribute
hour,minute,second and microsecond and
tzinfo(time zone information)

• Class datetime.date: have attribute year,
month and day.
• It is easy to create current date using
function today() method.

Brief introduction
• Standard library includes a large range of
tools working with files.
• The os module provides a way regardless the
operation systems.
• The glob module help scan the directory
contents

Work with file
• open:create, open, and modify files
• remove: delete files
Code Example:
import os
fi = open(file)
fo = open(temp,”w”) #w mean write permisson
for s in fi.readlines():
fo.write(s)
fi.close
fo.close
os.remove(back)

Work with directory
• listdir,chdir,mkdir,rmdir,getcwd: Please
guess the function by the name
import os
os.getpwd() # get the current dir
os.chdir(‘..’) # change to the parent directory
os.getcwd()
os.listdir(‘.’) #list the file under the dir
os.mkdir(‘./temp1’) #make new dir
os.rmdir(‘./temp1’) #delete the dir
os.listdir(‘.’) # check if the delete is successful

Work with directory - cont
• removedirs,makedirs:
remove and create directory hierarchies.
Instead, rmdir and mkdir only handle single
directory level.

Work with file attributes
• stat: It returns a 9-tuple which contains the
size, inode change timestamp, modification
timestamp, and access privileges of a file.
Similar as unix stat.
import os
file = "samples/sample.jpg“
st = os.stat(file)
size = st[6] #file size

Working with processes
• system:runs a new command under the
current process, and waits for it to finish
import os
os.system('dir')
os.system('notepad') # open notepad

The os.path class
• This module contains functions that deal with long filenames (path
names) in various ways.
• Learn from example
import os
filename = "my/little/pony"
print "using", os.name, "..."
print "split", "=>", os.path.split(filename)
print "splitext", "=>", os.path.splitext(filename)
print "dirname", "=>", os.path.dirname(filename)
print "basename", "=>", os.path.basename(filename)
print "join", "=>", os.path.join(os.path.dirname(filename),
os.path.basename(filename))

Using the os.path module to check
what a filename represents
• Learn from example
for file in FILES:
print file, "=>",
if os.path.exists(file):
print "EXISTS",
if os.path.isabs(file):
print "ISABS",
if os.path.isdir(file):
print "ISDIR",
if os.path.isfile(file):
print "ISFILE",
if os.path.islink(file):
print "ISLINK",
if os.path.ismount(file):
print "ISMOUNT",
print

os.environ
• A mapping object representing the string
environment. => key value pairs
a = os.environ
dir(a) # show all the functions of a
a.keys() #show all the keys
a.has_key('USERNAME') #check if has this key
print a['USERNAME‘] # return the value of this key

The glob module: search dir
• An asterisk(*) mathes 0 or more characters in
a segment of a name
>>> import glob
>>> for name in glob.glob(‘dir/*’)
print name

More wildcards in glob
• A question mark (?) matches any single
character
>>> for name in glob.glob(‘./file?.txt’):
print name
./file1.txt
./file2.txt
• Others: character range e.g. [a-z], [0-9]

The tempfile module: Temporary
file system object
• Application need temporary file to store
data.
• This module create temporary files with
unique names securely.
• The file is removed automatically when it is
closed.

Use TemporaryFile create temp
file
>>> import tempfile

Another example
• Write something into temp file.
• Use seek() back to the beginning of file. Then
read it

More methods in tempfile
• Method NamedTemporaryFile()
– Similar as TemporaryFile but it give a named
temporrary file.
– Leave it to user fig out (Follow the example of
TemporaryFile).

• Method mkdtemp(): create temp dir
• Method gettempdir(): return the default dir
store temp file

Module shutil – high level file
operation
• Method copyfile(source,destination): copy
source file to destination)

• Method copy(source file, dir): copy the file
under the dir

More functions in shutil
• Method copytree(dir1, dir2): copy a dir1 to
dir2
• Method rmtree(dir): remove a dir and its
contents.
• Method move(source,destination): move a
file or dir from one place to another.

Module filecmp: compare files
and dir
• Function filecmp.cmp(file1,file2): return True
or False
• Function filecmp.dircmp(dir1,dir2).report():
output a plain-text report

Brief introduction
• This module provides access to some
variables used or maintained by the
interpreter and to functions that interact
strongly with the interpreter.

Working with command-line arguments
• argv list contain the arguments passed to the script.
The first is the script itself (sys.argv[0])
# File:sys-argv-example-1.py
import sys
print "script name is", sys.argv[0]
for arg in sys.argv[1:]:
print arg

• Save the code to file sys-argv-example-1.py, run
command line “python sys-argv-example-1.py –c
option1 –d option2”

Working with modules
• path: The path list contains a list of directory
names where Python looks for extension
modules

import sys
sys.path

sys.platform
• The platform variable contains the name of
the host platform
import sys
sys.platform

• Typical platform names are win32 for Windows

Working with standard input and output
• The stdin, stdout and stderr variables contain
stream objects corresponding to the standard
I/O streams.
#File “test.py”
saveout = sys.stdout
f = open(‘file1.txt’,’w’)
Sys.stdout = f #change the stdout to file1.txt
print “hello,world”
sys.stdout = saveout
In this example, “hello,world” string has written to file1.txt.

sys.exit:Exiting the program
• This function takes an optional integer value,
which is returned to the calling program.
import sys
print "hello"
sys.exit(1)
print "there"

Python advanced 3.the python std lib by example – system related modules

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