right usage of repr() in python?

Question

I have a container class which I would like to use to print all the elements in it.

I'd like to print them to file or console.

I've laid out the element (Patch) and container class as below and __repr__(self).
I'm not sure I understood the purpose of __repr__() and am wondering whether the usage here is OK.

class Patch:
    def __init__(self, folder_name, file_name):
        self.folder_name = folder_name
        self.file_name = file_name
        self.full_path = os.path.join(self.folder_name, self.file_name)
        self.file_hash = md5_for_file(open(self.full_path, 'r'))
        self.file_size = os.path.getsize(self.full_path)

    def __repr__(self):
        return "%s %s %s" % (self.file_name, self.file_hash, self.file_size)

class PatchContainer:
    def __init__(self):
        self.patch_folder_dict = collections.OrderedDict()
        self.patch_file_set = set()
    def addPatch(self, patch):
        if patch.file_name in self.patch_file_set:
            print '*** Delete the file ', patch.full_path, ' ***'
            return
        self.patch_file_set.add(patch.file_name)
        if not patch.folder_name in self.patch_folder_dict:
            self.patch_folder_dict[patch.folder_name] = [patch]
        else:
            self.patch_folder_dict[patch.folder_name].append(patch)

    def prettyPrint(self, writeable_object=PATCH_META_FILE):
        sys.stdout = writeable_object
        for patch_folder in self.patch_folder_dict.keys():
            print patch_folder
            patch_list = self.patch_folder_dict[patch_folder]
            for patch in patch_list:
                print patch
        sys.stdout = sys.__stdout__

It works as intended but please comment on whether the style/usage are fine.

score 4 · Accepted Answer · answered Jun 11 '12 at 03:15

4

This looks like you probably want __str__. They are similar, but __str__ returns something intended for human consumption; __repr__ should return something intended for Python consumption - it should either be a Python expression that will evaluate to an equal object, or something in angle brackets like the default <classname object at id>. See the documentation for the builtin repr(object), which ends up calling object.__repr__():

For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval(), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information

answered Jun 11 '12 at 03:15

lvc

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Raymond Hettinger (@raymondh) tweeted on the subject the other day: "When possible, `__repr__` methods should show how an object was constructed. The invariant should be: `eval(repr(obj)) == obj`." – Ned Deily Jun 11 '12 at 03:26
how about prettyPrint() function? I could have defined PatchContainer's __str()__ instead of prettyPrint. – eugene Jun 12 '12 at 02:01
@Eugene yes. It would probably make more sense to do it that way, and so instead of calling `patchcontainer.prettyprint(file)` you would do `print >>file patchcontainer` (or `print(patchcontainer, file=file)` if you do `from __future__ import print_function`). It is almost always a good idea to separate business-level classes from the code that does IO. – lvc Jun 12 '12 at 03:05

score 2 · Answer 2 · edited May 23 '17 at 10:24

2

It's a human-readable representation of the object.

"Purpose of Python's __repr__": clicky
"Difference between __str__ and __repr__ in Python": clicky
Python documentation: clicky

edited May 23 '17 at 10:24

Community

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answered Jun 11 '12 at 03:15

Cat

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right usage of __repr__() in python?

2 Answers2

right usage of repr() in python?