MIME type and character set extraction¶
The zope.mimetype.typegetter
module provides a selection of MIME
type extractors (implementations of
zope.mimetype.interfaces.IMimeTypeGetter
) and charset extractors
(implementations of zope.mimetype.interfaces.ICharsetGetter
).
These may be used to determine what the MIME type and character set
for uploaded data should be.
These two interfaces represent the site policy regarding interpreting upload data in the face of missing or inaccurate input.
Let’s go ahead and import the module:
>>> from zope.mimetype import typegetter
MIME types¶
There are a number of interesting MIME-type extractors:
mimeTypeGetter()
- A minimal extractor that never attempts to guess.
mimeTypeGuesser()
- An extractor that tries to guess the content type based on the name and data if the input contains no content type information.
smartMimeTypeGuesser()
- An extractor that checks the content for a variety of constructs to
try and refine the results of the
mimeTypeGuesser()
. This is able to do things like check for XHTML that’s labelled as HTML in upload data.
mimeTypeGetter()
¶
We’ll start with the simplest, which does no content-based guessing at all, but uses the information provided by the browser directly. If the browser did not provide any content-type information, or if it cannot be parsed, the extractor simply asserts a “safe” MIME type of application/octet-stream. (The rationale for selecting this type is that since there’s really nothing productive that can be done with it other than download it, it’s impossible to mis-interpret the data.)
When there’s no information at all about the content, the extractor returns None:
>>> print(typegetter.mimeTypeGetter())
None
Providing only the upload filename or data, or both, still produces None, since no guessing is being done:
>>> print(typegetter.mimeTypeGetter(name="file.html"))
None
>>> print(typegetter.mimeTypeGetter(data=b"<html>...</html>"))
None
>>> print(typegetter.mimeTypeGetter(
... name="file.html", data=b"<html>...</html>"))
None
If a content type header is available for the input, that is used since that represents explicit input from outside the application server. The major and minor parts of the content type are extracted and returned as a single string:
>>> typegetter.mimeTypeGetter(content_type="text/plain")
'text/plain'
>>> typegetter.mimeTypeGetter(content_type="text/plain; charset=utf-8")
'text/plain'
If the content-type information is provided but malformed (not in conformance with RFC 2822), it is ignored, since the intent cannot be reliably guessed:
>>> print(typegetter.mimeTypeGetter(content_type="foo bar"))
None
This combines with ignoring the other values that may be provided as expected:
>>> print(typegetter.mimeTypeGetter(
... name="file.html", data=b"<html>...</html>", content_type="foo bar"))
None
mimeTypeGuesser()
¶
A more elaborate extractor that tries to work around completely
missing information can be found as the mimeTypeGuesser()
function.
This function will only guess if there is no usable content type
information in the input. This extractor can be thought of as having
the following pseudo-code:
def mimeTypeGuesser(name=None, data=None, content_type=None):
type = mimeTypeGetter(name=name, data=data, content_type=content_type)
if type is None:
type = guess the content type
return type
Let’s see how this affects the results we saw earlier. When there’s no input to use, we still get None:
>>> print(typegetter.mimeTypeGuesser())
None
Providing only the upload filename or data, or both, now produces a non-None guess for common content types:
>>> typegetter.mimeTypeGuesser(name="file.html")
'text/html'
>>> typegetter.mimeTypeGuesser(data=b"<html>...</html>")
'text/html'
>>> typegetter.mimeTypeGuesser(name="file.html", data=b"<html>...</html>")
'text/html'
Note that if the filename and data provided separately produce different MIME types, the result of providing both will be one of those types, but which is unspecified:
>>> mt_1 = typegetter.mimeTypeGuesser(name="file.html")
>>> mt_1
'text/html'
>>> mt_2 = typegetter.mimeTypeGuesser(data=b"<?xml version='1.0'?>...")
>>> mt_2
'text/xml'
>>> mt = typegetter.mimeTypeGuesser(
... data=b"<?xml version='1.0'?>...", name="file.html")
>>> mt in (mt_1, mt_2)
True
If a content type header is available for the input, that is used in
the same way as for the mimeTypeGetter()
function:
>>> typegetter.mimeTypeGuesser(content_type="text/plain")
'text/plain'
>>> typegetter.mimeTypeGuesser(content_type="text/plain; charset=utf-8")
'text/plain'
If the content-type information is provided but malformed, it is ignored:
>>> print(typegetter.mimeTypeGetter(content_type="foo bar"))
None
When combined with values for the filename or content data, those are still used to provide reasonable guesses for the content type:
>>> typegetter.mimeTypeGuesser(name="file.html", content_type="foo bar")
'text/html'
>>> typegetter.mimeTypeGuesser(
... data=b"<html>...</html>", content_type="foo bar")
'text/html'
Information from a parsable content-type is still used even if a guess from the data or filename would provide a different or more-refined result:
>>> typegetter.mimeTypeGuesser(
... data=b"GIF89a...", content_type="application/octet-stream")
'application/octet-stream'
smartMimeTypeGuesser()
¶
The smartMimeTypeGuesser()
function applies more knowledge to the
process of determining the MIME-type to use. Essentially, it takes
the result of the mimeTypeGuesser()
function and attempts to refine
the content-type based on various heuristics.
We still see the basic behavior that no input produces None:
>>> print(typegetter.smartMimeTypeGuesser())
None
An unparsable content-type is still ignored:
>>> print(typegetter.smartMimeTypeGuesser(content_type="foo bar"))
None
The interpretation of uploaded data will be different in at least some
interesting cases. For instance, the mimeTypeGuesser()
function
provides these results for some XHTML input data:
>>> typegetter.mimeTypeGuesser(
... data=b"<?xml version='1.0' encoding='utf-8'?><html>...</html>",
... name="file.html")
'text/html'
The smart extractor is able to refine this into more usable data:
>>> typegetter.smartMimeTypeGuesser(
... data=b"<?xml version='1.0' encoding='utf-8'?>...",
... name="file.html")
'application/xhtml+xml'
In this case, the smart extractor has refined the information determined from the filename using information from the uploaded data. The specific approach taken by the extractor is not part of the interface, however.
charsetGetter()
¶
If you’re interested in the character set of textual data, you can use
the charsetGetter
function (which can also be registered as the
ICharsetGetter
utility):
The simplest case is when the character set is already specified in the content type.
>>> typegetter.charsetGetter(content_type='text/plain; charset=mambo-42')
'mambo-42'
Note that the charset name is lowercased, because all the default ICharset and ICharsetCodec utilities are registered for lowercase names.
>>> typegetter.charsetGetter(content_type='text/plain; charset=UTF-8')
'utf-8'
If it isn’t, charsetGetter
can try to guess by looking at actual data
>>> typegetter.charsetGetter(content_type='text/plain', data=b'just text')
'ascii'
>>> typegetter.charsetGetter(content_type='text/plain', data=b'\xe2\x98\xba')
'utf-8'
>>> import codecs
>>> typegetter.charsetGetter(data=codecs.BOM_UTF16_BE + b'\x12\x34')
'utf-16be'
>>> typegetter.charsetGetter(data=codecs.BOM_UTF16_LE + b'\x12\x34')
'utf-16le'
If the character set cannot be determined, charsetGetter
returns None.
>>> typegetter.charsetGetter(content_type='text/plain', data=b'\xff')
>>> typegetter.charsetGetter()