# open

`asdf.``open`(fd, uri=None, mode=None, validate_checksums=False, extensions=None, do_not_fill_defaults=False, ignore_version_mismatch=True, ignore_unrecognized_tag=False, _force_raw_types=False, copy_arrays=False, lazy_load=True, custom_schema=None, strict_extension_check=False, ignore_missing_extensions=False, _compat=False)

  Open an existing ASDF file.

  Parameters:

  fd : string or file-like object

  > May be a string `file` or `http` URI, or a Python file-like object.

  uri : string, optional

  > The URI of the file. Only required if the URI can not be automatically determined from `fd`.

  mode : string, optional

  > The mode to open the file in. Must be `r` (default) or `rw`.

  validate_checksums : bool, optional

  > If `True`, validate the blocks against their checksums. Requires reading the entire file, so disabled by default.

  extensions : list of AsdfExtension

  > A list of extensions to use when reading and writing ASDF files. See `AsdfExtension` for more information.

  do_not_fill_defaults : bool, optional

  > When `True`, do not fill in missing default values.

  ignore_version_mismatch : bool, optional

  > When `True`, do not raise warnings for mismatched schema versions. Set to `True` by default.

  ignore_unrecognized_tag : bool, optional

  > When `True`, do not raise warnings for unrecognized tags. Set to `False` by default.

  copy_arrays : bool, optional

  > When `False`, when reading files, attempt to memmap underlying data arrays when possible.

  lazy_load : bool, optional

  > When `True` and the underlying file handle is seekable, data arrays will only be loaded lazily: i.e. when they are accessed for the first time. In this case the underlying file must stay open during the lifetime of the tree. Setting to False causes all data arrays to be loaded up front, which means that they can be accessed even after the underlying file is closed. Note: even if `lazy_load` is `False`, `copy_arrays` is still taken into account.

  custom_schema : str, optional

  > Path to a custom schema file that will be used for a secondary validation pass. This can be used to ensure that particular ASDF files follow custom conventions beyond those enforced by the standard.

  strict_extension_check : bool, optional

  > When `True`, if the given ASDF file contains metadata about the extensions used to create it, and if those extensions are not installed, opening the file will fail. When `False`, opening a file under such conditions will cause only a warning. Defaults to `False`.

  ignore_missing_extensions : bool, optional

  > When `True`, do not raise warnings when a file is read that contains metadata about extensions that are not available. Defaults to `False`.

  Returns:

  asdffile : AsdfFile

  > The new AsdfFile object.