Path Management

Goal

Normalize paths on different platform
Create, copy and remove folders
Handle errors

Modules



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import os
import os.path
import shutil
import errno
import glob
import sys

Multiplatform Path Management

The os.path module seems verbose but it's the best way to manage paths. It's:
- safe
- multiplatform

Here we check the operating system and prepend the right path



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import os
import sys
basedir, hosts  = "/", "etc/hosts"



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# sys.platform shows the current operating system
if sys.platform.startswith('win'):
    basedir = 'c:/windows/system32/drivers'
print(basedir)



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# Join removes redundant "/"
hosts = os.path.join(basedir, hosts)
print(hosts)



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# normpath fixes "/" orientation 
# and redundant ".."
hosts = os.path.normpath(hosts)
print("Normalized path is", hosts)



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# realpath resolves symlinks (on unix)
! mkdir -p /tmp/course
! ln -sf /etc/hosts /tmp/course/hosts
realfile = os.path.realpath("/tmp/course/hosts") 
print(realfile)



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# Exercise: given the following path
base, path = "/usr", "bin/foo"
# Which is the expected output of result?
result = os.path.join(base, path)
print(result)

Manage trees

Python modules can:

- manage directory trees
- and basic errors



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# os and shutil supports basic file operations
# like recursive copy and tree creation.
!rm -rf /tmp/course/foo
from os import makedirs
makedirs("/tmp/course/foo/bar")



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# while os.path can be used to test file existence
from os.path import isdir
assert isdir("/tmp/course/foo/bar")



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# Check the directory content with either one of
!tree /tmp/course || find /tmp/course



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# We can use exception handlers to check
#  what happened.

try:
    # python2 does not allow to ignore
    #  already existing directories
    #  and raises an OSError
    makedirs("/tmp/course/foo/bar")
except OSError as e:
    # Just use the errno module to
    #  check the error value
    print(e)
    import errno
    assert e.errno == errno.EEXIST



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from shutil import copytree, rmtree
# Now copy recursively two directories
# and check the result
copytree("/tmp/course/foo", "/tmp/course/foo2")
assert isdir("/tmp/course/foo2/bar")



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#This command should work on both unix and windows 
!ls /tmp/course/foo2/



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# Now remove it and check the outcome
rmtree("/tmp/course/foo")
assert not isdir("/tmp/course/foo/bar")



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#This command should work on both unix and windows 
!ls /tmp/course/



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# Cleanup created files
rmtree("/tmp/course")

Encoding

Goals

A string is more than a sequence of bytes
A string is a couple (bytes, encoding)
Use unicode_literals in python2
Manage differently encoded filenames
A string is not a sequence of bytes



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import os
import os.path
import glob



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from os.path import isdir
basedir = "/tmp/course"
if not isdir(basedir):
    os.makedirs(basedir)



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# Py3 doesn't need the 'u' prefix before the string.
the_string = u"S\u00fcd" # Sued
print(the_string)
print type(the_string)



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# the_string Sued can be encoded in different...
in_utf8 = the_string.encode('utf-8')
in_win = the_string.encode('cp1252')

# ...byte-sequences
assert type(in_utf8) == bytes
print type(in_utf8)



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# Now you can see the differences between 
print(repr(in_utf8))
# and
print(repr(in_win))



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# Decoding bytes using the wrong map...
# ...gives Sad results
print(in_utf8.decode('cp1252'))
print (in_utf8.decode('utf-8'))

To make Py2 encoding-aware we must

from future import unicode_literals, print_function

Path Management

Goal

Modules

See also:

Multiplatform Path Management

Manage trees

Encoding

Goals

To make Py2 encoding-aware we must