You are welcome to submit new loaders to core VisiData, or as plugins. Please, see our checklists for contribution.

Creating a new loader for a data source is simple and straigthforward.

  1. open_filetype boilerplate

  2. FooSheet subclass with rowtype and rowdef

  3. FooSheet reload or iterload

  4. FooSheet.columns

Hello Loader

Here’s a step-by-line breakdown of a basic loader, which reads in a text file as a series of lines. This same general structure and process should work for all loaders.

Step 1. open_<filetype> boilerplate

def open_readme(vd, p):
    return ReadmeSheet(p.base_stem, source=p)

This is used for filetype readme, which is used for files with extension .readme, or when specified manually with the filetype option like --filetype=readme or -f readme on the command line.

The open_<filetype> function usually looks exactly like this, with only the type of Sheet changed.

The p argument is a visidata.Path.

The actual loading happens in the Sheet. An existing sheet type can be used, or a new sheet type can be created.

Step 2. Create a Sheet subclass

class ReadmeSheet(TableSheet):
    rowtype = 'lines'   # rowdef: [str]
  • TableSheet (and its alias Sheet) is the basic tabular sheet of rows and columns. Most loader sheets will inherit from TableSheet, but some might inherit from more specialized sheets if they share functionality, or from BaseSheet if they are not tabular (like the Canvas).

  • The rowtype member is only displayed on the right-hand status. It should be plural. If not given, it is “rows”. It’s helpful to give the user an subconscious check of the kind of sheet being shown.

  • The rowdef should be given for all loaders, even though it is only a comment. It specifies the expected Pythonic structure of the rows on this sheet. This is important because nearly every other component of the sheet depends on this structure.

Step 3. Load data into rows, and yield them one-by-one

reload() is called when the Sheet is first pushed, and thereafter by the user with Ctrl+R. The default TableSheet.reload() iterates through the rows returned by TableSheet.iterload(), and takes care of a few common tasks (like running async and resetting the rows member to a new list).

Each loader for a tabular sheet should overload iterload(), which uses the Sheet source to populate and then yield each row one-by-one.

class ReadmeSheet(TableSheet):
    rowtype = 'lines'   # rowdef: [str]

    def iterload(self):
        for line in self.source:
            yield [line]


str by itself is not a valid rowdef.

Each row must have a unique rowid, which by default is the Python id() of the row. Because Python interns common strings, strings with the same value will have the same id. This would break a lot of features, like row selection for instance.

Also, as an immutable type, it would be annoying to not be able to modify it.

So it needs to be wrapped in a Python list, which is guaranteed to be unique, and also mutable.

  • sheet.source is the visidata.Path given as the source kwarg to ReadmeSheet() in open_readme.


Any kwarg passed to a Sheet constructor will be stored on the sheet in an attribute of the same name.


visidata.Path <vd-path> objects are Path-like but have some additional features, like being iterable (yielding their contents one line at a time).

While there is a visidata.Path.read_text() function, do not use for line in p.read_text().splitlines() in a loader, as that will read the entire file before returning the first line. A loader must be able to handle arbitrary amounts of data (including data too large to fit in memory), so this will not work.

Path.__iter__ is optimized to read the file a small amount at a time, so for line in path is workable for a textual line-based file format.

  • If the loader requires a third-party library, import it inside iterload() or reload() (or open_<filetype> if necessary). Do not import at the toplevel, or vd will fail to start when the library is not installed.

    • preferably, import it using modname = importExternal(modname, pythonPackageName. If the user does not have the package installed, it will output instructions to pip3 install pythonPackageName.

visidata.vd.importExternal(modname, pipmodname='')

By default, a Sheet has one Column which just displays a string representation of the row. So the above example is a good starting point for any loader; just get the rows however they come most easily from the source, and launch vd with a sample dataset in that format. Then use Ctrl+Y to explore the resulting Python object, to find what attributes to show on the sheet.


For more control over the whole loading process, BaseSheet.reload() can be overridden instead of iterload():

def reload(self):
    self.rows = []
    for line in self.source:
  • @asyncthread launches the decorated function in its own thread. See Threads

  • sheet.rows must always be reset to a new list. Never call sheet.rows.clear().

  • Always add rows using addRow().

Supporting asynchronous loaders

Loading a large dataset in the main thread will cause the interface to freeze. However, the basic TableSheet reload and iterload structure results in an asynchronous loader by default. Since rows are yielded one at a time, they become available as they are loaded, and reload itself is decorated with an @asyncthread, which causes it to be launched in a new thread.

  • All row iterators should be wrapped with Progress. This updates the progress percentage as it passes each element through.

  • Do not depend on the order of rows after they are added; e.g. do not reference rows[-1]. The order of rows may change during an asynchronous loader.

  • Catch any Exception that might be raised while handling a specific row, and add them as the row instead. Uncaught exceptions will cause the loader thread to abort.

  • Do not use a bare except: clause or the loader thread will not be cancelable with Ctrl+C.

Progress and Exception example

class FooSheet(Sheet):
    def iterload(self):
        for bar in Progress(foolib.iterfoo(self.source.open_text())):
                r = foolib.parse(bar)
            except Exception as e:
                r = e
            yield r

Testing for Loader Performance

Test the loader with a very large dataset to make sure that:

  • the first rows appear immediately;

  • the progress percentage is being updated;

  • the loader can be cancelled (with Ctrl+C).

Step 4. Enumerate the Columns

Each sheet has a columns attribute with a unique list of Column objects. Each Column provides a different view into the row.

class FooSheet(Sheet):
    rowtype = 'foobits'  # rowdef: foolib.Bar object

    columns = [
        ColumnAttr('name'),  # foolib.Bar.name
        Column('bar', getter=lambda col,row: row.inside[2],
                      setter=lambda col,row,val: row.set_bar(val)),
        Column('baz', type=int, getter=lambda col,row: row.inside[1]*100)

In general, set columns as a class member containing a list of static columns. If the columns aren’t known until data is loaded, reload/iterload can add new columns using addColumn().

If the rowdef is a list, and the columns are dynamic, SequenceSheet.reload() could handle the Column creation.

class FooSheet(SequenceSheet):
    rowtype = 'foobits'  # rowdef: a list, which is a sequence of values

    def iterload(self):
        with foolib.iterfoo(self.source.open_text() as f:
            r = foolib.parse(bar)
            yield r

Column attributes

Columns have several attributes; all except name are optional arguments to the constructor:

  • name: should be a valid Python identifier and unique among the column names on the sheet. (Otherwise the column cannot be used in an expression.)

  • type: can be str, int, float, date, currency, or a custom type. By default it is anytype, which passes the original value through unmodified.

  • width: the initial width for the column. 0 means hidden; None (default) means calculate on first draw.

Column getters can be any function, but many loaders are satisfied with a static list of ItemColumn (for values in dict and list rowdefs) and/or AttrColumn (for a members or attributes directly on the row object). This is dependent on the loader function; some loaders may prefer to do less parsing to load faster, and then the Columns will need to be correspondingly more complicated.

See the Columns section for a complete API.

Passthrough options

Loaders which use a Python library (internal or external) are encouraged to pass its kwargs using **options.getall("foo_") interface. For modules like csv which expose them as kwargs to some function or constructor, this is very easy:

rdr = csv.reader(fp, **csvoptions())

Full Example

This is a completely functional loader for the sas7bdat (SAS dataset file) format, thanks to Jared Hobbs’ sas7bdat package.

from visidata import Sheet, ItemColumn, Progress

def open_sas7bdat(vd, p):
    return SasSheet(p.base_stem, source=p)

class SasSheet(Sheet):
    def iterload(self):
        import sas7bdat
        SASTypes = { 'string': str, 'number': float, }

        self.dat = sas7bdat.SAS7BDAT(str(self.source),

        self.columns = []
        for col in self.dat.columns:
                                     type=SASTypes.get(col.type, anytype)))

        with self.dat as fp:
            yield from Progress(fp, total=self.dat.properties.row_count)

Guessing Filetypes

When loading a file, VisiData tries to infer its filetype by peeking at the initial lines of the file and guessing from its structure.

vd.guess_<filetype>(path) contains this logic for checking whether a file might be <filetype.

If those structures are not present, the function should return nothing. If they are, the function should return a dictionary with:

  • filetype being the filetype they detect (corresponding to the vd.open_<filetype>)

  • _likelihood (optional) being a number from 0-10, 10 being most likely and 0 meaning a last ditch effort if nothing else will take it

  • any other key/values will be set as options on the Sheet the open_<filetype> function returns

Examples of guess_filetype functions

def guess_foo(vd, p):
    import foobar
    if p.open_text().read(8).startswith("#Foo"):
        enc = foobar.encoding(p)
        return dict(filetype='foo', foo_encoding=enc)


A full-duplex loader requires a saver. The saver iterates over all rows and visibleCols, calling getValue, getDisplayValue or getTypedValue as the saving format allows, and saves the results in its format to the given path. Savers should be decorated with @VisiData.api in order to make them available through the vd object’s scope.

visidata.vd.save_txt(p, *vsheets)
  • p is a visidata.Path object referencing the file being written to.

  • sheets is a list of 1 or more sheets to be saved.

The saver should preserve the column names and translate their types into foolib semantics, but other attributes on the Columns are generally not saved.

Savers which can handle typed values should use Column.getTypedValue, and displayable savers (like html, markdown, csv) should use Column.getDisplayValue (which takes into account the column’s fmtstr).

With this example, saving as filetype table will call the tabulate library to save the data in any number of text formats, specified by the tbl_tablefmt option. (Several built-in savers use tabulate also, but those savers work a little differently, as each tablefmt is available as a direct save filetype.)

vd.option('tbl_tablefmt', 'simple', 'file format to save with "table" filetype')

def get_rows(sheet, cols):
    for row in Progress(sheet.rows):
        yield [ col.getDisplayValue(row) for col in cols ]

def save_table(path, *sheets):
    import tabulate

    with path.open_text(mode='w') as fp:
        for vs in sheets:
                get_rows(vs, vs.visibleCols),
                headers=[ col.name for col in vs.visibleCols ],


visidata.Path is a wrapper around Python’s builtin pathlib.Path that can also handle non-filesystem files (URLs, stdin, files within archives).

The given attribute is new to visidata.Path. Other functions listed here are wrappers around the equivalent pathlib.Path functions, with specialized functionality as needed for non-filesystem files. All other accesses are forwarded to the inner pathlib.Path object, but will probably not work for non-filesystem files.


The path as given to the constructor.


Whether this path exists.

visidata.Path.open(self, mode='rt', encoding=None, encoding_errors=None, newline=None)

Open path in text or binary mode, using options.encoding and options.encoding_errors. Return open file-pointer or file-pointer-like.

visidata.Path.open_text(self, mode='rt', encoding=None, encoding_errors=None, newline=None)

Open path in text or binary mode, using options.encoding and options.encoding_errors. Return open file-pointer or file-pointer-like.

visidata.Path.read_text(self, encoding=None, errors=None)

Open the file in text mode, read it, and close the file.

visidata.Path.open_bytes(self, mode='rb')

Open the file pointed by this path and return a file object in binary mode.


Return the entire binary contents of the pointed-to file as a bytes object.


Return the result of the stat() system call on this path, like os.stat() does.

visidata.Path.with_name(self, name)

Return a sibling Path with name as a filename in the same directory.

URL Scheme Loaders

When VisiData tries to open a URL with schemetype of foo (i.e. starting with foo://), it calls openurl_foo(urlpath, filetype). urlpath is a UrlPath object, with attributes for each of the elements of the parsed URL.

openurl_foo should return a Sheet or call error(). If the URL indicates a particular type of Sheet (like magnet://), then it should construct that Sheet itself. If the URL is just a means to get to another filetype, then it can call openSource with a Path-like object that knows how to fetch the URL:

def openurl_foo(p, filetype=None):
    return openSource(FooPath(p.url), filetype=filetype)