Extract file extension: Difference between revisions

and one way of allowing for OS-specific variations in the character sets permitted in file extensions is to write a general and reusable curried function, from which we can obtain simpler OS-specific functions by specialisation:

<lang python>'''Obtaining OS-specific file extensions'''

import os

import re

# OS-INDEPENDENT CURRIED FUNCTION -------------------------

# takeExtension :: Regex String -> FilePath -> String

def takeExtension(charSet):

'''The extension part (if any) of a file name.

(Given a regex string specification of the

character set accepted in extensions by the OS).'''

def go(fp):

m = re.search(

r'\.[' + charSet + ']+$',

(fp).split(os.sep)[-1]

)

return m[0] if m else ''

return lambda fp: go(fp)

# DERIVED (OS-SPECIFIC) FUNCTIONS -------------------------

# takePosixExtension :: FilePath -> String

def takePosixExtension(fp):

'''The file extension, if any,

of a Posix file path.'''

return takeExtension(r'A-Za-z0-9\-\_')(fp)

# takeWindowsExtension :: FilePath -> String

def takeWindowsExtension(fp):

'''The file extension, if any,

of a Windows file path.'''

return takeExtension(r'A-Za-z0-9')(fp)

# TEST ----------------------------------------------------

def main():

'''Tests'''

for f in [takePosixExtension, takeWindowsExtension]:

print(

tabulated(f.__name__ + ' :: FilePath -> String:')(

str

)(str)(f)([

"http://example.com/download.tar.gz",

"CharacterModel.3DS",

".desktop",

"document",

"document.txt_backup",

"/etc/pam.d/login"

])

)

print()

# GENERIC -------------------------------------------------

# tabulated :: String -> (a -> String) ->

# (b -> String) ->

# (a -> b) -> [a] -> String

def tabulated(s):

'''Heading -> x display function -> fx display function ->

number of columns -> f -> value list -> tabular string.'''

def go(xShow, fxShow, f, xs):

w = max(map(lambda x: len(xShow(x)), xs))

return s + '\n' + '\n'.join([

xShow(x).rjust(w, ' ') + ' -> ' + fxShow(f(x)) for x in xs

])

return lambda xShow: lambda fxShow: (

lambda f: lambda xs: go(

xShow, fxShow, f, xs

)

)

# MAIN ---

if __name__ == '__main__':

main()</lang>

{{Out}}

<pre>takePosixExtension :: FilePath -> String:

http://example.com/download.tar.gz -> .gz

CharacterModel.3DS -> .3DS

.desktop -> .desktop

document ->

document.txt_backup -> .txt_backup

/etc/pam.d/login ->

takeWindowsExtension :: FilePath -> String:

http://example.com/download.tar.gz -> .gz

CharacterModel.3DS -> .3DS

.desktop -> .desktop

document ->

document.txt_backup ->

/etc/pam.d/login -> </pre>