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This recipe provides a quick and easy way to process Java properties files using pure Python. Of course, Jython can always be used, but in situations where Jython cannot be used, this recipe provides a sure-fire drop-in replacement. The Properties class is modelled to duplicate the behaviour of the original as closely as possible.

Python, 317 lines
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#! /usr/bin/env python

"""
A Python replacement for java.util.Properties class
This is modelled as closely as possible to the Java original.

Created - Anand B Pillai <abpillai@gmail.com>    
"""

import sys,os
import re
import time

class IllegalArgumentException(Exception):

    def __init__(self, lineno, msg):
        self.lineno = lineno
        self.msg = msg

    def __str__(self):
        s='Exception at line number %d => %s' % (self.lineno, self.msg)
        return s
                 
class Properties(object):
    """ A Python replacement for java.util.Properties """
    
    def __init__(self, props=None):

        # Note: We don't take a default properties object
        # as argument yet

        # Dictionary of properties.
        self._props = {}
        # Dictionary of properties with 'pristine' keys
        # This is used for dumping the properties to a file
        # using the 'store' method
        self._origprops = {}

        # Dictionary mapping keys from property
        # dictionary to pristine dictionary
        self._keymap = {}
        
        self.othercharre = re.compile(r'(?<!\\)(\s*\=)|(?<!\\)(\s*\:)')
        self.othercharre2 = re.compile(r'(\s*\=)|(\s*\:)')
        self.bspacere = re.compile(r'\\(?!\s$)')
        
    def __str__(self):
        s='{'
        for key,value in self._props.items():
            s = ''.join((s,key,'=',value,', '))

        s=''.join((s[:-2],'}'))
        return s

    def __parse(self, lines):
        """ Parse a list of lines and create
        an internal property dictionary """

        # Every line in the file must consist of either a comment
        # or a key-value pair. A key-value pair is a line consisting
        # of a key which is a combination of non-white space characters
        # The separator character between key-value pairs is a '=',
        # ':' or a whitespace character not including the newline.
        # If the '=' or ':' characters are found, in the line, even
        # keys containing whitespace chars are allowed.

        # A line with only a key according to the rules above is also
        # fine. In such case, the value is considered as the empty string.
        # In order to include characters '=' or ':' in a key or value,
        # they have to be properly escaped using the backslash character.

        # Some examples of valid key-value pairs:
        #
        # key     value
        # key=value
        # key:value
        # key     value1,value2,value3
        # key     value1,value2,value3 \
        #         value4, value5
        # key
        # This key= this value
        # key = value1 value2 value3
        
        # Any line that starts with a '#' is considerered a comment
        # and skipped. Also any trailing or preceding whitespaces
        # are removed from the key/value.
        
        # This is a line parser. It parses the
        # contents like by line.

        lineno=0
        i = iter(lines)

        for line in i:
            lineno += 1
            line = line.strip()
            # Skip null lines
            if not line: continue
            # Skip lines which are comments
            if line[0] == '#': continue
            # Some flags
            escaped=False
            # Position of first separation char
            sepidx = -1
            # A flag for performing wspace re check
            flag = 0
            # Check for valid space separation
            # First obtain the max index to which we
            # can search.
            m = self.othercharre.search(line)
            if m:
                first, last = m.span()
                start, end = 0, first
                flag = 1
                wspacere = re.compile(r'(?<![\\\=\:])(\s)')        
            else:
                if self.othercharre2.search(line):
                    # Check if either '=' or ':' is present
                    # in the line. If they are then it means
                    # they are preceded by a backslash.
                    
                    # This means, we need to modify the
                    # wspacere a bit, not to look for
                    # : or = characters.
                    wspacere = re.compile(r'(?<![\\])(\s)')        
                start, end = 0, len(line)
                
            m2 = wspacere.search(line, start, end)
            if m2:
                # print 'Space match=>',line
                # Means we need to split by space.
                first, last = m2.span()
                sepidx = first
            elif m:
                # print 'Other match=>',line
                # No matching wspace char found, need
                # to split by either '=' or ':'
                first, last = m.span()
                sepidx = last - 1
                # print line[sepidx]
                
                
            # If the last character is a backslash
            # it has to be preceded by a space in which
            # case the next line is read as part of the
            # same property
            while line[-1] == '\\':
                # Read next line
                nextline = i.next()
                nextline = nextline.strip()
                lineno += 1
                # This line will become part of the value
                line = line[:-1] + nextline

            # Now split to key,value according to separation char
            if sepidx != -1:
                key, value = line[:sepidx], line[sepidx+1:]
            else:
                key,value = line,''

            self.processPair(key, value)
            
    def processPair(self, key, value):
        """ Process a (key, value) pair """

        oldkey = key
        oldvalue = value
        
        # Create key intelligently
        keyparts = self.bspacere.split(key)
        # print keyparts

        strippable = False
        lastpart = keyparts[-1]

        if lastpart.find('\\ ') != -1:
            keyparts[-1] = lastpart.replace('\\','')

        # If no backspace is found at the end, but empty
        # space is found, strip it
        elif lastpart and lastpart[-1] == ' ':
            strippable = True

        key = ''.join(keyparts)
        if strippable:
            key = key.strip()
            oldkey = oldkey.strip()
        
        oldvalue = self.unescape(oldvalue)
        value = self.unescape(value)
        
        self._props[key] = value.strip()

        # Check if an entry exists in pristine keys
        if self._keymap.has_key(key):
            oldkey = self._keymap.get(key)
            self._origprops[oldkey] = oldvalue.strip()
        else:
            self._origprops[oldkey] = oldvalue.strip()
            # Store entry in keymap
            self._keymap[key] = oldkey
        
    def escape(self, value):

        # Java escapes the '=' and ':' in the value
        # string with backslashes in the store method.
        # So let us do the same.
        newvalue = value.replace(':','\:')
        newvalue = newvalue.replace('=','\=')

        return newvalue

    def unescape(self, value):

        # Reverse of escape
        newvalue = value.replace('\:',':')
        newvalue = newvalue.replace('\=','=')

        return newvalue    
        
    def load(self, stream):
        """ Load properties from an open file stream """
        
        # For the time being only accept file input streams
        if type(stream) is not file:
            raise TypeError,'Argument should be a file object!'
        # Check for the opened mode
        if stream.mode != 'r':
            raise ValueError,'Stream should be opened in read-only mode!'

        try:
            lines = stream.readlines()
            self.__parse(lines)
        except IOError, e:
            raise

    def getProperty(self, key):
        """ Return a property for the given key """
        
        return self._props.get(key,'')

    def setProperty(self, key, value):
        """ Set the property for the given key """

        if type(key) is str and type(value) is str:
            self.processPair(key, value)
        else:
            raise TypeError,'both key and value should be strings!'

    def propertyNames(self):
        """ Return an iterator over all the keys of the property
        dictionary, i.e the names of the properties """

        return self._props.keys()

    def list(self, out=sys.stdout):
        """ Prints a listing of the properties to the
        stream 'out' which defaults to the standard output """

        out.write('-- listing properties --\n')
        for key,value in self._props.items():
            out.write(''.join((key,'=',value,'\n')))

    def store(self, out, header=""):
        """ Write the properties list to the stream 'out' along
        with the optional 'header' """

        if out.mode[0] != 'w':
            raise ValueError,'Steam should be opened in write mode!'

        try:
            out.write(''.join(('#',header,'\n')))
            # Write timestamp
            tstamp = time.strftime('%a %b %d %H:%M:%S %Z %Y', time.localtime())
            out.write(''.join(('#',tstamp,'\n')))
            # Write properties from the pristine dictionary
            for prop, val in self._origprops.items():
                out.write(''.join((prop,'=',self.escape(val),'\n')))
                
            out.close()
        except IOError, e:
            raise

    def getPropertyDict(self):
        return self._props

    def __getitem__(self, name):
        """ To support direct dictionary like access """

        return self.getProperty(name)

    def __setitem__(self, name, value):
        """ To support direct dictionary like access """

        self.setProperty(name, value)
        
    def __getattr__(self, name):
        """ For attributes not found in self, redirect
        to the properties dictionary """

        try:
            return self.__dict__[name]
        except KeyError:
            if hasattr(self._props,name):
                return getattr(self._props, name)
            
if __name__=="__main__":
    p = Properties()
    p.load(open('test2.properties'))
    p.list()
    print p
    print p.items()
    print p['name3']
    p['name3'] = 'changed = value'
    print p['name3']    
    p['new key'] = 'new value'
    p.store(open('test2.properties','w'))

A requirement in a Java-Python porting project led me to writing this class. The original Java code was working with property files a lot and the specification mentioned Jython could not be used. This led me to write this class and over a week or so, to make it adapt as closely as possible to the original.

The class can process property files which have either '=', ':' or space character as the separation character between name and value pairs. It can take into account a list of values also. Serializing functionality is also there.

This could be useful for pure Python projects which still want to process property files.

Hope this is useful.

[Update 27 July 06] - Allow changed properties to be serialized.

10 comments

Alex Greif 16 years, 2 months ago  # | flag

why don't you support storing changed properties? In the store() method always the _origProps are stored. In this case it does not behave like java Properties.

N B 15 years, 11 months ago  # | flag

Referencing previous properties. Thanks, the code was helpful. I noticed it did not allow you to reference previous properties like java does, e.g.:

prop1=hello
prop2={prop1} world!

I added code to handle this, if you want to add it: [in processPair, where value gets set]

value = self.unescape(value)

curlies = re.compile("{.+?}")
found = curlies.findall(value)

for f in found:
    srcKey = f[1:-1]
    if self._props.has_key(srcKey):
        value = value.replace(f, self._props[srcKey], 1)
Jussi Kuosa 15 years, 7 months ago  # | flag

Unicode properties? Hi, this was really helpful, but how do I get a unicode value from a properties file? I have a Java properties file that has been transformed into escaped unicode in Latin-1 (foo=b\u0061r). When I read that property into Python, my value string is 'b\u0061r'.

Has anyone tried this before?

Ellis Pritchard 13 years, 3 months ago  # | flag

Executing:

>>> props = Properties()
>>> 'a' in props

hangs:

^CTraceback (most recent call last):
File "<stdin>", line 1, in <module>
File "Properties.py", line 290, in __getitem__
  return self.getProperty(name)
File "Properties.py", line 237, in getProperty
  def getProperty(self, key):

whereas

>>> p['a']
''

and

>>> p.has_key('a')
False

work as expected... any clues? (Python newbie!)

ved 12 years, 7 months ago  # | flag

im a newbie in python. I'm using pydev plugin of eclipse to edit and run this script. When i run, i'm getting this error:

UnboundLocalError: local variable 'wspacere' referenced before assignment

Any clue? -Ved

ved 12 years, 7 months ago  # | flag

i could figure out the problem. The property file was having section header because of which it was failing.

fred bloggs 12 years, 2 months ago  # | flag

Hi thanks for the item, very useful. I'd like to post a change to 'setProperty()' to allow unicode...

def setProperty(self, key, value):
    """ Set the property for the given key """
#   was...
#   if type(key) is str and type(value) is str:
    if isinstance(key, basestring) and isinstance(value, basestring):
        self.processPair(key, value)
    else:
        raise TypeError,'both key and value should be (base)strings!'

Not saying it's the only change needed, just what I've come across so far. (Python newbie)

Raffi Mohammed 11 years, 4 months ago  # | flag

Anand,

Why not use ConfigParser ?

Thanks,

Raffi

Matt Good 10 years, 12 months ago  # | flag

@Raffi, ConfigParser handles .ini-style files, which follow a similar format to Java's properties files, but the two are not the same. ConfigParser expects section headers, which Java properties files do not have.

For those interested, I've written an alternative parser for Java properties files: http://mgood.github.com/jprops/

It adds some features missing from this implementation, most notably it provides full unicode support.

I've written a little more about it here: http://blog.matt-good.net/2011/10/07/released-jprops/

Anand B Pillai 10 years, 6 months ago  # | flag

Folks, this is no longer developed actively. Jesse noller has created a project from this recipe with some fixes not available here. I recommend that project to anyone using this recipe.

http://pypi.python.org/pypi/pyjavaproperties

--Anand