How to "generalize" a function?

[=?iso-8859-1?q?Thomas_K=F6llmann?=]

Hi, everybody!

I'm teaching myself Python, and I have no experience in programming
apart from some years of shell scripting. So, please bear with me.

These two funktions are part of an administrative script I've set
myself as a first lesson, and, as you will see, they're practically the same,
except for one variable. So I'd like to weld them together -- but I
can't find out how to.

def writeIP(ip):
""" IP schreiben """
regex = re.compile('(.*)address(.*)')
confFile = open(networkConf, 'r')
conf = confFile.readlines()
confFile.close
for line in conf:
if regex.search(line):
addressLine = line
addressLineNum = conf.index(addressLine)
address = string.split(addressLine, ' ')
address[1] = ip + "\n"
conf[addressLineNum] = string.join(address)
confFile = open(networkConf, 'w')
confFile.writelines(conf)
confFile.close

def writeMask(mask):
""" netmask schreiben """
regex = re.compile('(.*)netmask(.*)')
confFile = open(networkConf, 'r')
conf = confFile.readlines()
confFile.close
for line in conf:
if regex.search(line):
netmaskLine = line
netmaskLineNum = conf.index(netmaskLine)
netmask = string.split(netmaskLine, ' ')
netmask[1] = mask + "\n"
conf[netmaskLineNum] = string.join(netmask)
confFile = open(networkConf, 'w')
confFile.writelines(conf)
confFile.close

I feel it should be possible to use something like

def writeFile(ip,keyword):
...

but how would I construct expressions like

netmaskLineNum = conf.index(netmaskLine)

in that case (netmask being the keyword here)?


I fear this is a really silly question and I guess I've missing a very
basic concept here. But I can't figure it out alone. Any advice much
appreciated!

Mit schönem Gruß
- Thomas

 

[Dan Sommers]

Hi, everybody!
I'm teaching myself Python, and I have no experience in programming
apart from some years of shell scripting. So, please bear with me.

These two funktions are part of an administrative script I've set
myself as a first lesson, and, as you will see, they're practically
the same, except for one variable. So I'd like to weld them together
-- but I can't find out how to.

[ two very similar functions snipped ]

I feel it should be possible to use something like

def writeFile(ip,keyword):
...
Absolutely.

but how would I construct expressions like

netmaskLineNum = conf.index(netmaskLine)

in that case (netmask being the keyword here)?

netmaskLineNum and addressLineNum are just names. They may mean
something to you and to people who read your program, but they mean
nothing to Python. So just use generic names:

for line in conf:
if regex.search( line )
theLine = line
theLineNum = conf.index( theLine )

etc.

HTH,
Dan

 

[Alexander Schmolck]

confFile.close

You want ``confFile.close()`` -- the above won't do anything [1].

'as


Footnotes:
[1] Best practice would be something like this (don't worry to much about it
-- it just ensures the file is properly closed, even if something goes
wrong):

confFile = None
try:
confFile = open(networkConf, 'w')
confFile.writelines(conf)
finally:
if confFile: confFile.close()

 

[Michael Spencer]

Hi, everybody!

I'm teaching myself Python, and I have no experience in programming
apart from some years of shell scripting. So, please bear with me.

These two funktions are part of an administrative script I've set
myself as a first lesson, and, as you will see, they're practically the same,
except for one variable. So I'd like to weld them together -- but I
can't find out how to.

def writeIP(ip):
""" IP schreiben """
regex = re.compile('(.*)address(.*)')

This is the only difference between the functions, isn't it?
So, instead of hardwiring 'address' or 'netmask' into the regexp template, you
should insert it based on an argument passed to the function. String
interpolation works well here: e.g.,

confFile = open(networkConf, 'r')
conf = confFile.readlines()
confFile.close

Note, here you presumably mean confFile.close() i.e., you must supply the parens
to call the function.


[snip]

I feel it should be possible to use something like

def writeFile(ip,keyword):
...

Indeed. Use keyword as the argument to the string interpolation

but how would I construct expressions like

netmaskLineNum = conf.index(netmaskLine)

I think these should work unchanged. But it would be easier to read if you
changed these names to be neutral to the application e.g., instead of
netmaskLine, foundLine

HTH

Michael

 

[Scott David Daniels]

[1] Best practice would be something like this (don't worry to much about it
-- it just ensures the file is properly closed, even if something goes
wrong):

confFile = None
try:
confFile = open(networkConf, 'w')
confFile.writelines(conf)
finally:
if confFile: confFile.close()

A clearer equivalent is:
confFile = open(networkConf, 'w')
try:
confFile.writelines(conf)
finally:
confFile.close()


You did not say whether you are looking to replace all, the first,
or the last occurrence of your search target. Assuming you really
mean all lines and the first occurrence on those lines:

def replaces(source, search, replacement):
'''Replace first of search on lines with replacement'''
pat = re.compile(search)
for line in source:
yield re.sub(pat, replacement, line, 1)

And your application might go like:

input = open(networkConf, 'r')
part1 = replaces(input, 'address', 'ip')
part2 = replaces(part1, 'netmask', 'mask')
result = list(part2)
input.close()
output = open(networkConf, 'w')
try:
output.writelines(result)
finally:
output.close()


--Scott David Daniels
(e-mail address removed)

 

[jfj]

Hi, everybody!

I'm teaching myself Python, and I have no experience in programming
apart from some years of shell scripting. So, please bear with me.

These two funktions are part of an administrative script I've set
myself as a first lesson, and, as you will see, they're practically the same,
except for one variable. So I'd like to weld them together -- but I
can't find out how to.

Pass the variable as an argument probably.
But because generally you wouldn't want to recompile the regexp (this
should be done once), you could say:

# untested
def makewriter (regexp_string):
def writeFunc(ip, regex=re.compile(regexp_string)):
confFile = open(networkConf, 'r')
conf = confFile.readlines()
confFile.close
for line in conf:
if regex.search(line):
addressLine = line
addressLineNum = conf.index(addressLine)
address = string.split(addressLine, ' ')
address[1] = ip + "\n"
conf[addressLineNum] = string.join(address)
confFile = open(networkConf, 'w')
confFile.writelines(conf)
confFile.close
return writeFunc

writeIP=makewriter('(.*)address(.*)')
writeMask=makewriter('(.*)netmask(.*)')

This is rather advanced python programming though, but it shows
cool dynamic function creation features and it's never early to
get into it

jfj

 

[=?iso-8859-1?q?Thomas_K=F6llmann?=]

This is the only difference between the functions, isn't it?
So, instead of hardwiring 'address' or 'netmask' into the regexp
template, you should insert it based on an argument passed to the
function. String interpolation works well here: e.g.,

Thanks, I somehow missed this (presumably very basic) feature.

Note, here you presumably mean confFile.close() i.e., you must supply
the parens to call the function.

Yes, thanks -- to Alexander as well, who also pointed me to that!

Mit schönem Gruß
- Thomas

 

[=?iso-8859-1?q?Thomas_K=F6llmann?=]

On Sun, 24 Apr 2005 23:40:22 +0200,

netmaskLineNum and addressLineNum are just names. They may mean
something to you and to people who read your program, but they mean
nothing to Python. So just use generic names:

Thanks. I still get easily confused when trying to "abstract" such
things, but I guess I'll get it some fine day. I appreciate you took
the time to point that out to me -- you must have found it _extremely_
obvious.

Mit schönem Gruß
- Thomas