Programming Forums > Script Programming > Python

MR.T · Jun 8th, 2007, 2:02 PM

I have been programming in python off and on and I am trying to really sit down and get more proficient in it this summer.

Right now I am trying to make a basic program for reading, modifying and writing data with an XML file. I am using minidom and I and here is the problem I am running into. Say I have a game inventory or something like this:

(Toggle Plain Text)

<Inv>
	<Sword>
		<Name>Broad Sword</Name>
	</Sword>
	<Shield>
		<Name>Big Shield</Name>
	</Shield>
</Inv>

I want to return the Name section of Shield. If I use a function like "getElementsByTagName" then it will return a list containing the contents of all Name sections. How do I explicitly tell minidom to get an element in... say this order Inv>Shield>Name? I tried setting it up so it loaded the elements in that order i.e I loaded Inv into a variable then I loaded Shield and finally Name. That works fine if the document is small, but obviously that's inefficient, and I cant save the file if I choose to modify an element.

I'm sure there is something simple that I am forgetting or that I missed(there always seems to be in python). Can someone please steer me in the right direction.

Arevos · Jun 9th, 2007, 9:03 AM

 python Syntax (Toggle Plain Text) 
import xml.dom.minidom
xml = xml.dom.minidom.parseString("""<Inv>
...     <Sword>
...             <Name>Broad Sword</Name>
...     </Sword>
...     <Shield>
...             <Name>Big Shield</Name>
...     </Shield>
... </Inv>""")
 
# Assume only one inventory
inv = xml.getElementsByTagName("Inv")[0]
 
# Get all shields in inventory
shields = inv.getElementsByTagName("Shield")
 
# For each shield, print out its name
for shield in shields:
    # Assume shield has only one name
    name = shield.getElementsByTagName("Name")[0]
 
    # Assume name tag just has text in it
    print name.firstChild.wholeText
import xml.dom.minidom
xml = xml.dom.minidom.parseString("""<Inv>
...     <Sword>
...             <Name>Broad Sword</Name>
...     </Sword>
...     <Shield>
...             <Name>Big Shield</Name>
...     </Shield>
... </Inv>""")

# Assume only one inventory
inv = xml.getElementsByTagName("Inv")[0]

# Get all shields in inventory
shields = inv.getElementsByTagName("Shield")

# For each shield, print out its name
for shield in shields:
    # Assume shield has only one name
    name = shield.getElementsByTagName("Name")[0]

    # Assume name tag just has text in it
    print name.firstChild.wholeText

Generally speaking though, there's easier ways of storing human-readable game data in Python, unless you happen to want your data to be cross-platform.

MR.T · Jun 9th, 2007, 1:19 PM

Well what you posted is essentially what I did do read data from an XML file. However my problem really is about modifying the file. Say I want to change the "Broad Sword" to "Short Sword" or add another element to my XML file. I could do it that way, but wouldn't the changes just take place to the code that was in the variable? How do I modify and save the document.

Arevos · Jun 9th, 2007, 5:03 PM

Well, say you have found the element you want via the approach demonstrated above, and assigned it to the variable "sword":

 python Syntax (Toggle Plain Text) 
# Get name element of sword
name = sword.getElementsByTagName("Name")[0]
 
# Change text in name
name.firstChild.replaceWholeText("Short Sword")
 
# Output the new xml
print xml.toxml()
# Get name element of sword
name = sword.getElementsByTagName("Name")[0]

# Change text in name
name.firstChild.replaceWholeText("Short Sword")

# Output the new xml
print xml.toxml()

This demonstrates that one can alter the XML DOM tree in place, and then output it as XML with all the changes made.

However, this isn't a very good way of doing things. A better way would be to have a "dump" and "load" method on each class:

 python Syntax (Toggle Plain Text) 
import xml.dom.minidom as dom
 
class Sword:
    def __init__(self, name):
        self.name = name
 
    def dump(self):
        sword = dom.Element("Sword")
        name = dom.Element("Name")
        text = dom.Text()
        text.replaceWholeText(self.name)
        name.appendChild(text)
        sword.appendChild(name)
        return sword
 
    @classmethod
    def load(self, sword_element):
        name = sword_element.getElementsByTagName("Name")[0]
        return Sword(name.firstChild.wholeText)
import xml.dom.minidom as dom

class Sword:
    def __init__(self, name):
        self.name = name

    def dump(self):
        sword = dom.Element("Sword")
        name = dom.Element("Name")
        text = dom.Text()
        text.replaceWholeText(self.name)
        name.appendChild(text)
        sword.appendChild(name)
        return sword

    @classmethod
    def load(self, sword_element):
        name = sword_element.getElementsByTagName("Name")[0]
        return Sword(name.firstChild.wholeText)

But we can do even better than this:

 python Syntax (Toggle Plain Text) 
import xml.dom.minidom as dom
 
def todom(sexpr):
    if type(sexpr) is list or type(sexpr) is tuple:
        node = dom.Element(sexpr[0])
        for child in sexpr[1:]:
            node.appendChild(todom(child))
    else:
        node = dom.Text()
        node.replaceWholeText(str(sexpr))
    return node
import xml.dom.minidom as dom

def todom(sexpr):
    if type(sexpr) is list or type(sexpr) is tuple:
        node = dom.Element(sexpr[0])
        for child in sexpr[1:]:
            node.appendChild(todom(child))
    else:
        node = dom.Text()
        node.replaceWholeText(str(sexpr))
    return node

The above function converts s-expressions into XML, and we can use this to considerably shorten the code of our Sword class:

 python Syntax (Toggle Plain Text) 
class Sword:
    def __init__(self, name):
        self.name = name
 
    def dump(self):
        return todom(("Sword" ("Name", self.name)))
 
    @classmethod
    def load(self, sword_element):
        name = sword_element.getElementsByTagName("Name")[0]
        return Sword(name.firstChild.wholeText)
class Sword:
    def __init__(self, name):
        self.name = name

    def dump(self):
        return todom(("Sword" ("Name", self.name)))

    @classmethod
    def load(self, sword_element):
        name = sword_element.getElementsByTagName("Name")[0]
        return Sword(name.firstChild.wholeText)

There's probably quite a few XML serialisation libraries worth looking into as well... Though I'd personally look into YAML as an alternative to storing human-editable data to file.

MR.T · Jun 9th, 2007, 11:16 PM

Ok I see, thank you very much for the informative post

Jun 8th, 2007, 2:02 PM	#1
MR.T Newbie Join Date: Feb 2006 Location: USA,Michigan Posts: 22 Rep Power: 0	XML minidom... help I have been programming in python off and on and I am trying to really sit down and get more proficient in it this summer. Right now I am trying to make a basic program for reading, modifying and writing data with an XML file. I am using minidom and I and here is the problem I am running into. Say I have a game inventory or something like this: (Toggle Plain Text) <Inv> <Sword> <Name>Broad Sword</Name> </Sword> <Shield> <Name>Big Shield</Name> </Shield> </Inv> <Inv> <Sword> <Name>Broad Sword</Name> </Sword> <Shield> <Name>Big Shield</Name> </Shield> </Inv> I want to return the Name section of Shield. If I use a function like "getElementsByTagName" then it will return a list containing the contents of all Name sections. How do I explicitly tell minidom to get an element in... say this order Inv>Shield>Name? I tried setting it up so it loaded the elements in that order i.e I loaded Inv into a variable then I loaded Shield and finally Name. That works fine if the document is small, but obviously that's inefficient, and I cant save the file if I choose to modify an element. I'm sure there is something simple that I am forgetting or that I missed(there always seems to be in python). Can someone please steer me in the right direction. __________________ "Gee, Brain, what do you want to do tonight?" "The same thing we do every night, Pinky: Try to take over the world!"

Jun 9th, 2007, 9:03 AM	#2
Arevos Programming Guru Join Date: Aug 2005 Location: England Posts: 1,499 Rep Power: 5	python Syntax (Toggle Plain Text) import xml.dom.minidom xml = xml.dom.minidom.parseString("""<Inv> ... <Sword> ... <Name>Broad Sword</Name> ... </Sword> ... <Shield> ... <Name>Big Shield</Name> ... </Shield> ... </Inv>""") # Assume only one inventory inv = xml.getElementsByTagName("Inv")[0] # Get all shields in inventory shields = inv.getElementsByTagName("Shield") # For each shield, print out its name for shield in shields: # Assume shield has only one name name = shield.getElementsByTagName("Name")[0] # Assume name tag just has text in it print name.firstChild.wholeText import xml.dom.minidom xml = xml.dom.minidom.parseString("""<Inv> ... <Sword> ... <Name>Broad Sword</Name> ... </Sword> ... <Shield> ... <Name>Big Shield</Name> ... </Shield> ... </Inv>""") # Assume only one inventory inv = xml.getElementsByTagName("Inv")[0] # Get all shields in inventory shields = inv.getElementsByTagName("Shield") # For each shield, print out its name for shield in shields: # Assume shield has only one name name = shield.getElementsByTagName("Name")[0] # Assume name tag just has text in it print name.firstChild.wholeText Generally speaking though, there's easier ways of storing human-readable game data in Python, unless you happen to want your data to be cross-platform.

Jun 9th, 2007, 1:19 PM	#3
MR.T Newbie Join Date: Feb 2006 Location: USA,Michigan Posts: 22 Rep Power: 0	Well what you posted is essentially what I did do read data from an XML file. However my problem really is about modifying the file. Say I want to change the "Broad Sword" to "Short Sword" or add another element to my XML file. I could do it that way, but wouldn't the changes just take place to the code that was in the variable? How do I modify and save the document. __________________ "Gee, Brain, what do you want to do tonight?" "The same thing we do every night, Pinky: Try to take over the world!"

Jun 9th, 2007, 5:03 PM	#4
Arevos Programming Guru Join Date: Aug 2005 Location: England Posts: 1,499 Rep Power: 5	Well, say you have found the element you want via the approach demonstrated above, and assigned it to the variable "sword": python Syntax (Toggle Plain Text) # Get name element of sword name = sword.getElementsByTagName("Name")[0] # Change text in name name.firstChild.replaceWholeText("Short Sword") # Output the new xml print xml.toxml() # Get name element of sword name = sword.getElementsByTagName("Name")[0] # Change text in name name.firstChild.replaceWholeText("Short Sword") # Output the new xml print xml.toxml() This demonstrates that one can alter the XML DOM tree in place, and then output it as XML with all the changes made. However, this isn't a very good way of doing things. A better way would be to have a "dump" and "load" method on each class: python Syntax (Toggle Plain Text) import xml.dom.minidom as dom class Sword: def __init__(self, name): self.name = name def dump(self): sword = dom.Element("Sword") name = dom.Element("Name") text = dom.Text() text.replaceWholeText(self.name) name.appendChild(text) sword.appendChild(name) return sword @classmethod def load(self, sword_element): name = sword_element.getElementsByTagName("Name")[0] return Sword(name.firstChild.wholeText) import xml.dom.minidom as dom class Sword: def __init__(self, name): self.name = name def dump(self): sword = dom.Element("Sword") name = dom.Element("Name") text = dom.Text() text.replaceWholeText(self.name) name.appendChild(text) sword.appendChild(name) return sword @classmethod def load(self, sword_element): name = sword_element.getElementsByTagName("Name")[0] return Sword(name.firstChild.wholeText) But we can do even better than this: python Syntax (Toggle Plain Text) import xml.dom.minidom as dom def todom(sexpr): if type(sexpr) is list or type(sexpr) is tuple: node = dom.Element(sexpr[0]) for child in sexpr[1:]: node.appendChild(todom(child)) else: node = dom.Text() node.replaceWholeText(str(sexpr)) return node import xml.dom.minidom as dom def todom(sexpr): if type(sexpr) is list or type(sexpr) is tuple: node = dom.Element(sexpr[0]) for child in sexpr[1:]: node.appendChild(todom(child)) else: node = dom.Text() node.replaceWholeText(str(sexpr)) return node The above function converts s-expressions into XML, and we can use this to considerably shorten the code of our Sword class: python Syntax (Toggle Plain Text) class Sword: def __init__(self, name): self.name = name def dump(self): return todom(("Sword" ("Name", self.name))) @classmethod def load(self, sword_element): name = sword_element.getElementsByTagName("Name")[0] return Sword(name.firstChild.wholeText) class Sword: def __init__(self, name): self.name = name def dump(self): return todom(("Sword" ("Name", self.name))) @classmethod def load(self, sword_element): name = sword_element.getElementsByTagName("Name")[0] return Sword(name.firstChild.wholeText) There's probably quite a few XML serialisation libraries worth looking into as well... Though I'd personally look into YAML as an alternative to storing human-editable data to file.

Jun 9th, 2007, 11:16 PM	#5
MR.T Newbie Join Date: Feb 2006 Location: USA,Michigan Posts: 22 Rep Power: 0	Ok I see, thank you very much for the informative post __________________ "Gee, Brain, what do you want to do tonight?" "The same thing we do every night, Pinky: Try to take over the world!"

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Programming Forums > Script Programming > Python

XML minidom... help