Usage

Trollsift include collection of modules that assist with formatting, parsing and filtering satellite granule file names. These modules are useful and necessary for writing higher level applications and api’s for satellite batch processing. Currently we are implementing the string parsing and composing functionality. Watch this space for further modules to do with various types of filtering of satellite data granules.

Parser

The trollsift string parser module is useful for composing (formatting) and parsing strings compatible with the Python Format String Syntax. In satellite data file name filtering, the library is useful for extracting typical information from granule filenames, such as observation time, platform and instrument names. The trollsift Parser can also verify that the string formatting is invertible, i.e. specific enough to ensure that parsing and composing of strings are bijective mappings ( aka one-to-one correspondence ) which may be essential for some applications, such as predicting granule

parsing

The Parser object holds a format string, allowing us to parse and compose strings:

>>> from trollsift import Parser
>>>
>>> p = Parser("/somedir/{directory}/hrpt_{platform:4s}{platnum:2s}_{time:%Y%m%d_%H%M}_{orbit:05d}.l1b")
>>> data = p.parse("/somedir/otherdir/hrpt_noaa16_20140210_1004_69022.l1b")
>>> print(data) # doctest: +NORMALIZE_WHITESPACE
{'directory': 'otherdir', 'platform': 'noaa', 'platnum': '16',
 'time': datetime.datetime(2014, 2, 10, 10, 4), 'orbit': 69022}

Parsing in trollsift is not “greedy”. This means that in the case of ambiguous patterns it will match the shortest portion of the string possible. For example:

>>> from trollsift import Parser
>>>
>>> p = Parser("{field_one}_{field_two}")
>>> data = p.parse("abc_def_ghi")
>>> print(data)
{'field_one': 'abc', 'field_two': 'def_ghi'}

So even though the first field could have matched to “abc_def”, the non-greedy parsing chose the shorter possible match of “abc”.

composing

The reverse operation is called ‘compose’, and is equivalent to the Python string class format method. Here we take the filename pattern from earlier, change the time stamp of the data, and write out a new file name,

>>> from datetime import datetime
>>> data['time'] = datetime(2012, 1, 1, 1, 1)
>>> p.compose(data)
'/somedir/otherdir/hrpt_noaa16_20120101_0101_69022.l1b'

In addition to python’s builtin string formatting functionality trollsift also provides extra conversion options such as making all characters lowercase:

>>> my_parser = Parser("{platform_name!l}")
>>> my_parser.compose({'platform_name': 'NPP'})
'npp'

For all of the options see StringFormatter.

standalone parse and compose

The parse and compose methods also exist as standalone functions, depending on your requirements you can call,

>>> from trollsift import parse, compose
>>> fmt = "/somedir/{directory}/hrpt_{platform:4s}{platnum:2s}_{time:%Y%m%d_%H%M}_{orbit:05d}.l1b"
>>> data = parse( fmt, "/somedir/otherdir/hrpt_noaa16_20140210_1004_69022.l1b" )
>>> data['time'] = datetime(2012, 1, 1, 1, 1)
>>> compose(fmt, data)
'/somedir/otherdir/hrpt_noaa16_20120101_0101_69022.l1b'

And achieve the exact same result as in the Parse object example above.