Usage

Let’s demonstrate the main features of FileFinder using a simple example. Detailed information about some steps will be provided in separate pages.

We are going to deal with a dataset with multiple files all located in the directory /data/. They are organized by sub-directories corresponding to different parameter values, then in yearly sub-directories:

/data/param_[parameter]/[year]/variable_[date].nc
/data/param_0.0/2012/variable_2012-01-01.nc
/data/param_0.0/2012/variable_2012-01-02.nc
...
/data/param_1.5/2012/variable_2012-01-01.nc
...

Create the Finder object

To manage this, we are going to use the main entry point of this package: the Finder class. Its main arguments are the root directory containing the files, and a pattern specifying the filename structure. That pattern allows to get filenames corresponding to given values, but also to scan for files matching the pattern on disk.

finder = Finder(
    "/data/",
    "param_%(param:fmt=.1f)/%(Y)/variable_%(Y)-%(m)-%(d).nc"
)

The parts that vary from file to file are indicated in the pattern by parentheses, preceded by a percent sign. Within the parentheses are specifications for a Group, that will handle creating the regular expression to find files and formatting values appropriately.

Important

Details on the different ways to specify a varying group are available there: Pattern.

Here quickly, for date related parts, we only need to indicate the name: filefinder has them as default. For the parameter, indicating a string format will suffice.

Fix groups

Each group can be fixed to one possible value or a set of possible values. This will restrict the filenames that match the pattern when scanning files.

Note

Also, when creating filenames, if a group already has a fixed value it will be used by default.

Fixing groups can be done with either the Finder.fix_group() or Finder.fix_groups() methods. Groups can be selected either by their index in the filename pattern (starting from 0), or by their name. If using a group name, multiple groups can be fixed to the same value at once.

The given value can be:

• a number: will be formatted to a string according to the group specification. For scanning files, the string will be properly escaped for use in a regular expression.

• a boolean: if the group has two options (specified with the bool keyword), one of the options is selected and used as a string.

• a string: the value is directly interpreted as a regular expression and used as-is when scanning files or creating filenames, without further escaping or formatting.

• a list of any of the above: each element will be formatted to a string if not already. When scanning files, all elements are considered by joining them with OR ((value1|value2|...)), and when creating files only the first element of the list is used.

So for example:

>>> finder.fix_group("param", "[a-z]+")
will be kept as is
>>> finder.fix_group("param", 3.)
will be formatted as "3\.0"

More practically, we could keep only the files corresponding to january:

finder.fix_groups("m", 1)

We could also select specific days using a list:

finder.fix_groups(d=[1, 3, 5, 7])

Note

Fixed values can be changed/overwritten at any time, or unfixed using the Finder.unfix_groups() method.

Warning

A group flagged as :discard will not be fixed by default, unless using the keyword argument fix_discard in fix_group() and fix_groups().

Find files

Retrieve files

Files can be retrieved with the Finder.get_files() method, or from the Finder.files attribute. Both will automatically scan the directory for matching files and cache the results for future accesses. The files are stored in alphabetical order.

Note

The cache is appropriately voided when using some methods, like for fixing groups. For that reason, avoid setting attributes directly and use set methods.

The method get_files() simply returns a sorted list of the filenames found when scanning. By default the full path is returned, ie the concatenation of the root directory and the pattern part. It can also return the filename relative to the root directory (ie only the pattern part).

Instead of a flat list of filenames, get_files() can also arrange them in nested lists. To that end, one must provide the nested argument with a list that specify the order in which groups must be nested. Each element of the list gives:

• a group, by index or name, so that files be grouped together based on the value of that group

• multiple groups, by a tuple of indices or names, so files are grouped based on the combination of values from those groups.

An example might help to grasp this. Again with the same pattern, we can ask to group by values of ‘param’:

>>> finder.get_files(nested=["param"])
[
  [
    "/data/param_0.0/2012-01-01.nc",
    "/data/param_0.0/2012-01-02.nc",
    ...
  ],
  [
    "/data/param_1.5/2012-01-01.nc",
    "/data/param_1.5/2012-01-02.nc",
    ...
  ],
  ...
]

We obtain as many lists as different values found for ‘param’. Because we did not specify any other group, the nesting stop there. But we could chose to also group by the year:

>>> finder.get_files(nested=["param", "Y"])
[
  [  # param = 0
    [  # Y = 2012
      "/data/param_0.0/2012-01-01.nc",
      ...
    ],
    [  # Y = 2013
      "/data/param_0.0/2013-01-01.nc",
      ...
    ],
    ...
  ],
  [  # param = 1.5
    ...
  ],
  ...
]

Or if we wanted to group by date as well we can specify multiple groups for one nesting level:

>>> finder.get_files(nested=["param", ("Y", "m", "d")])
[
  [  # param = 0
    ["/data/param_0.0/2012-01-01.nc"],
    ["/data/param_0.0/2012-01-02.nc"],
    ...
  ],
  [  # param = 1.5
    ["/data/param_1.5/2012-01-01.nc"],
    ["/data/param_1.5/2012-01-02.nc"],
    ...
  ],
  ...
]

Note

This is aimed to work with xarray.open_mfdataset, which will merge files in a specific order when supplied a nested list of files.

Retrieve information

As some metadata might only be found in the filenames, FileFinder offer the possibility to retrieve it easily. The Finder caches a list of files matching the pattern, along with information about parts that matched the groups.

The Finder.files attribute stores a list of tuples each containing a filename and a Matches object storing that information.

Note

One can also scan any filename for matches with the Finder.find_matches() function.

For most cases, the simplest is to access the Matches object with a group index or name:

>>> file, matches = finder.files[0]
>>> matches["param"]
0.0  # a float, parsed from the filename

This method has several caveats:

• When using a group name, the first group in the pattern with that name is taken, even if there could be more groups with different values (a warning is issued if that is the case).

• Only groups not flagged as ‘:discard’ will be selected. If no group can be found, an error will be raised.

• The parsing of a value from the filename can fail for a variety of reasons, if that is the case, an error will be raised.

To counter those, one can use Matches.get_values() which will return a list of values corresponding to the selected group(s). It has arguments keep_discard and parse to choose whether keep discarded groups and whether to use the parsed value or solely the string that matched.

Matches.get_value() will return the first element of that list, raise if the list is empty, and warn if the values are not all equal.

Note

matches[key] is a thin wrapper around matches.get_value(key, parse=True, keep_discard=False).

As date/time values are scattered among multiple groups, the package supply the function library.get_date() to easily retrieve a datetime object from matches:

from filefinder.library import get_date
matches = finder.get_matches(filename)
date = get_date(matches)

Directories in pattern

The pattern can contain directory separators. The Finder can explore sub-directories to find the files.

Important

In the pattern, a directory separator should always be indicated with the forward slash /, even on Windows where we normally use the backslash. It will be replaced by the correct character when necessary.

We do this because the backslash has special meanings in regular expressions, and it is difficult to disambiguate the two.

The scanning process is as follows. It first generates a regular expression based on the pattern and the fixed values. This expression is meant to match paths relative to the root directory and have a capturing group for each pattern group.

The Finder then explore all sub-directories to find matching files using one of two methods.

1. By default, the regular expression is split at each path separator occurrence, so that we can eliminate folders that do not match the pattern. However, it cannot deal with some patterns in which a group contains a path separator.

2. For those more complicated patterns, by setting the attribute/parameter Finder.scan_everything to true, we will explore all sub-directories up to a depth of Finder.max_scan_depth.

The second method can be more costly for some directory structures —with many siblings folders for instance— but can deal with more exotic patterns. A likely example could be that of an optional directory:

>>> "basedir/%(subdir:bool=subdir_name/:)rest_of_pattern"
basedir/rest_of_pattern
basedir/subdir_name/rest_of_pattern

Create filenames

Using the information contained in the filename pattern we can also generate arbitrary filenames. This is done with Finder.make_filename(). Any group that does not already have its value fixed must have a value supplied as argument. As for fixing, a value will be appropriately formatted but a string will be left untouched.

So for instance:

>>> finder.make_filename(param=1.5, Y=2012, m=1, d=5)
"/data/param_1.5/2012-01-05.nc"

we can also fix some groups:

>>> finder.fix_groups(param=2., Y=2014)
>>> finder.make_filename(m=5, d=1)
"/data/param_2.0/2014-05-01.nc"
>>> finder.make_filename(m=6, d=1)
"/data/param_2.0/2014-06-01.nc"

and also supply a string to forgo formatting:

>>> finder.make_filename(param="this-feels-wrong", m=6, d=1)
"/data/param_this-feels-wrong/2014-06-01.nc"