Exercise 4.2
In this exercise, we’re simply going to look at some of the modules used for performing common tasks on the system such as working with files, directories, launching subprocesses, and so forth. These are tasks that might be normally written in shell scripts.
(a) Manipulating filenames
Start by putting a filename in a string:
>>> filename = 'Data/portfolio.csv'
>>>
In many programs, you might need to perform various actions involving
file and pathnames. More often than not, you use the os.path
module for this. Let’s try some operations:
>>> import os.path
>>> os.path.basename(filename) # Get basic filename
'portfolio.csv'
>>> os.path.dirname(filename) # Get the path
'Data'
>>> os.path.abspath(filename) # Get absolute path (output varies)
'/Users/beazley/Desktop/practical-python/Data/portfolio.csv'
>>>
Let’s probe the file a bit:
>>> os.path.exists(filename)
True
>>> os.path.isfile(filename)
True
>>> os.path.isdir(filename)
False
>>> os.path.getsize(filename)
126L
>>> os.path.getmtime(filename)
1248789706.0
>>> import time
>>> time.ctime(os.path.getmtime(filename))
'Sun Jan 19 05:34:52 2014'
>>>
Let’s change the filename so that it’s in a different directory.
>>> os.path.join('NewData', os.path.basename(filename))
'NewData/portfolio.csv'
>>>
(b) Finding files on the file system
The library module glob has a function glob() that
returns a list of filenames that match a given shell pattern. For example,
to get a list of all .py files in the current directory, you
do this:
>>> import glob
>>> glob.glob('*.py')
[ 'bounce.py', 'hello.py', 'mortgage.py', 'sears.py', ... ]
>>>
If you need to search through many directories, use the
os.walk() function. Try this example which prints all
Python files under the current working directory:
>>> import os
>>> for path, dirs, files in os.walk('.'):
for filename in files:
if filename.endswith('.py'):
print os.path.join(path, filename)
... look at the output ...
>>>
(c) Running a Subprocess
Use the subprocess module to run the external command "netstat -a" and collect the output in a string. This should work on both Unix and Windows.
>>> import subprocess
>>> out = subprocess.check_output(['netstat','-a'])
>>> print out
... you should see the output of netstat -a here ...
>>>
Note: If you’re using Python 2.6 or earlier, you’ll need to use this instead:
>>> import subprocess
>>> p = subprocess.Popen(['netstat','-a'],stdout=subprocess.PIPE)
>>> out = p.stdout.read()
>>> print out
... you should see the output of netstat -a here ...
>>>
(d) Command Line Options
Many real-world Python programs run as programs from the command line.
Modify your report.py program so that it accepts the two filenames
on the command line. For example:
bash % python report.py Data/portfolio.csv Data/prices.csv
Name Shares Price Change
---------- ---------- ---------- ----------
AA 100 9.22 -22.98
IBM 50 106.28 15.18
CAT 150 35.46 -47.98
MSFT 200 20.89 -30.34
GE 95 13.48 -26.89
MSFT 50 20.89 -44.21
IBM 100 106.28 35.84
bash %