[Python-Dev] Re: Printing objects on files (original) (raw)

Ka-Ping Yee ping@lfw.org
Wed, 3 May 2000 02:51:30 -0700 (PDT)

On Wed, 3 May 2000, Ka-Ping Yee wrote:

Fantasizing about other useful kinds of state beyond "encs" and "floatprec" ("listmax"? "ratprec"?) and managing this namespace is left as an exercise to the reader.

Okay, i lied. Shortly after writing this i realized that it is probably advisable for all such bits of state to be stored in stacks, so an interface such as this might do:

def push(self, key, value):
    if not self.state.has_key(key):
        self.state[key] = []
    self.state[key].append(value)

def pop(self, key):
    if self.state.has_key(key):
        if len(self.state[key]):
            self.state[key].pop()

def get(self, key):
    if not self.state.has_key(key):
        stack = self.state[key][-1]
    if stack:
        return stack[-1]
    return None

Thus:

>>> print 1/3
0.33333333333333331

>>> sys.stdout.push("float.prec", 6)
>>> print 1/3
0.333333

>>> sys.stdout.pop("float.prec")
>>> print 1/3
0.33333333333333331

And once we allow arbitrary strings as keys to the bits of state, the period is a natural separator we can use for managing the namespace.

Take the special case for Unicode out of the file object:

def printout(self, x):
    x.__print__(self)
    self.write("\n")

and have the Unicode string do the work:

def __printon__(self, file):
    file.write(self.encode(file.get("unicode.enc")))

This behaves just right if an encoding of None means ASCII.

If mucking with encodings is sufficiently common, you could imagine conveniences on file objects such as

def __init__(self, filename, mode, encoding=None):
    ...
    if encoding:
        self.push("unicode.enc", encoding)

def pushenc(self, encoding):
    self.push("unicode.enc", encoding)

def popenc(self, encoding):
    self.pop("unicode.enc")

-- ?!ng