Headers

Header

class astropy.io.fits.Header(cards=[], copy=False)

Bases: object

FITS header class. This class exposes both a dict-like interface and a list-like interface to FITS headers.

The header may be indexed by keyword and, like a dict, the associated value will be returned. When the header contains cards with duplicate keywords, only the value of the first card with the given keyword will be returned. It is also possible to use a 2-tuple as the index in the form (keyword, n)–this returns the n-th value with that keyword, in the case where there are duplicate keywords.

For example:

>>> header['NAXIS']
0
>>> header[('FOO', 1)]  # Return the value of the second FOO keyword
'foo'

The header may also be indexed by card number:

>>> header[0]  # Return the value of the first card in the header
'T'

Commentary keywords such as HISTORY and COMMENT are special cases: When indexing the Header object with either ‘HISTORY’ or ‘COMMENT’ a list of all the HISTORY/COMMENT values is returned:

>>> header['HISTORY']
This is the first history entry in this header.
This is the second history entry in this header.
...

See the Astropy documentation for more details on working with headers.

Notes

Although FITS keywords must be exclusively upper case, retrieving an item in a Header object is case insensitive.

Construct a Header from an iterable and/or text file.

Parameters:

cardslist of Card, optional

The cards to initialize the header with. Also allowed are other Header (or dict-like) objects.

Changed in version 1.2: Allowed cards to be a dict-like object.

copybool, optional

If True copies the cards if they were another Header instance. Default is False.

New in version 1.3.

add_blank(value='', before=None, after=None)

Add a blank card.

Parameters:

valuestr, optional

Text to be added.

beforestr or int, optional

Same as in Header.update

afterstr or int, optional

Same as in Header.update

add_comment(value, before=None, after=None)

Add a COMMENT card.

Parameters:

valuestr

Text to be added.

beforestr or int, optional

Same as in Header.update

afterstr or int, optional

Same as in Header.update

add_history(value, before=None, after=None)

Add a HISTORY card.

Parameters:

valuestr

History text to be added.

beforestr or int, optional

Same as in Header.update

afterstr or int, optional

Same as in Header.update

append(card=None, useblanks=True, bottom=False, end=False)

Appends a new keyword+value card to the end of the Header, similar to list.append.

By default if the last cards in the Header have commentary keywords, this will append the new keyword before the commentary (unless the new keyword is also commentary).

Also differs from list.append in that it can be called with no arguments: In this case a blank card is appended to the end of the Header. In the case all the keyword arguments are ignored.

Parameters:

cardstr, tuple

A keyword or a (keyword, value, [comment]) tuple representing a single header card; the comment is optional in which case a 2-tuple may be used

useblanksbool, optional

If there are blank cards at the end of the Header, replace the first blank card so that the total number of cards in the Header does not increase. Otherwise preserve the number of blank cards.

bottombool, optional

If True, instead of appending after the last non-commentary card, append after the last non-blank card.

endbool, optional

If True, ignore the useblanks and bottom options, and append at the very end of the Header.

property cards

The underlying physical cards that make up this Header; it can be looked at, but it should not be modified directly.

clear()

Remove all cards from the header.

property comments

View the comments associated with each keyword, if any.

For example, to see the comment on the NAXIS keyword:

>>> header.comments['NAXIS']
number of data axes

Comments can also be updated through this interface:

>>> header.comments['NAXIS'] = 'Number of data axes'

copy(strip=False)

Make a copy of the Header.

Changed in version 1.3: copy.copy and copy.deepcopy on a Header will call this method.

Parameters:

stripbool, optional

If True, strip any headers that are specific to one of the standard HDU types, so that this header can be used in a different HDU.

Returns:

Header

A new Header instance.

count(keyword)

Returns the count of the given keyword in the header, similar to list.count if the Header object is treated as a list of keywords.

Parameters:

keywordstr

The keyword to count instances of in the header

property data_size

Return the size (in bytes) of the data portion following the Header.

property data_size_padded

Return the size (in bytes) of the data portion following the Header including padding.

extend(cards, strip=True, unique=False, update=False, update_first=False, useblanks=True, bottom=False, end=False)

Appends multiple keyword+value cards to the end of the header, similar to list.extend.

Parameters:

cardsiterable

An iterable of (keyword, value, [comment]) tuples; see Header.append.

stripbool, optional

Remove any keywords that have meaning only to specific types of HDUs, so that only more general keywords are added from extension Header or Card list (default: True).

uniquebool, optional

If True, ensures that no duplicate keywords are appended; keywords already in this header are simply discarded. The exception is commentary keywords (COMMENT, HISTORY, etc.): they are only treated as duplicates if their values match.

updatebool, optional

If True, update the current header with the values and comments from duplicate keywords in the input header. This supersedes the unique argument. Commentary keywords are treated the same as if unique=True.

update_firstbool, optional

If the first keyword in the header is ‘SIMPLE’, and the first keyword in the input header is ‘XTENSION’, the ‘SIMPLE’ keyword is replaced by the ‘XTENSION’ keyword. Likewise if the first keyword in the header is ‘XTENSION’ and the first keyword in the input header is ‘SIMPLE’, the ‘XTENSION’ keyword is replaced by the ‘SIMPLE’ keyword. This behavior is otherwise dumb as to whether or not the resulting header is a valid primary or extension header. This is mostly provided to support backwards compatibility with the old Header.fromTxtFile method, and only applies if update=True.

useblanks, bottom, endbool, optional

These arguments are passed to Header.append() while appending new cards to the header.

classmethod fromfile(fileobj, sep='', endcard=True, padding=True)

Similar to Header.fromstring(), but reads the header string from a given file-like object or filename.

Parameters:

fileobjstr, file-like object

A filename or an open file-like object from which a FITS header is to be read. For open file handles the file pointer must be at the beginning of the header.

sepstr, optional

The string separating cards from each other, such as a newline. By default there is no card separator (as is the case in a raw FITS file).

endcardbool, optional

If True (the default) the header must end with an END card in order to be considered valid. If an END card is not found an OSError is raised.

paddingbool, optional

If True (the default) the header will be required to be padded out to a multiple of 2880, the FITS header block size. Otherwise any padding, or lack thereof, is ignored.

Returns:

Header

A new Header instance.

classmethod fromkeys(iterable, value=None)

Similar to dict.fromkeys()–creates a new Header from an iterable of keywords and an optional default value.

This method is not likely to be particularly useful for creating real world FITS headers, but it is useful for testing.

Parameters:

iterable

Any iterable that returns strings representing FITS keywords.

valueoptional

A default value to assign to each keyword; must be a valid type for FITS keywords.

Returns:

Header

A new Header instance.

classmethod fromstring(data, sep='')

Creates an HDU header from a byte string containing the entire header data.

Parameters:

datastr or bytes

String or bytes containing the entire header. In the case of bytes they will be decoded using latin-1 (only plain ASCII characters are allowed in FITS headers but latin-1 allows us to retain any invalid bytes that might appear in malformatted FITS files).

sepstr, optional

The string separating cards from each other, such as a newline. By default there is no card separator (as is the case in a raw FITS file). In general this is only used in cases where a header was printed as text (e.g. with newlines after each card) and you want to create a new Header from it by copy/pasting.

Returns:

Header

A new Header instance.

Examples

>>> from astropy.io.fits import Header
>>> hdr = Header({'SIMPLE': True})
>>> Header.fromstring(hdr.tostring()) == hdr
True

If you want to create a Header from printed text it’s not necessary to have the exact binary structure as it would appear in a FITS file, with the full 80 byte card length. Rather, each “card” can end in a newline and does not have to be padded out to a full card length as long as it “looks like” a FITS header:

>>> hdr = Header.fromstring("""\
... SIMPLE  =                    T / conforms to FITS standard
... BITPIX  =                    8 / array data type
... NAXIS   =                    0 / number of array dimensions
... EXTEND  =                    T
... """, sep='\n')
>>> hdr['SIMPLE']
True
>>> hdr['BITPIX']
8
>>> len(hdr)
4

classmethod fromtextfile(fileobj, endcard=False)

Read a header from a simple text file or file-like object.

Equivalent to:

>>> Header.fromfile(fileobj, sep='\n', endcard=False,
...                 padding=False)

See also

fromfile

get(key, default=None)

Similar to dict.get()–returns the value associated with keyword in the header, or a default value if the keyword is not found.

Parameters:

keystr

A keyword that may or may not be in the header.

defaultoptional

A default value to return if the keyword is not found in the header.

Returns:

value: str, number, complex, bool, or astropy.io.fits.card.Undefined

The value associated with the given keyword, or the default value if the keyword is not in the header.

index(keyword, start=None, stop=None)

Returns the index if the first instance of the given keyword in the header, similar to list.index if the Header object is treated as a list of keywords.

Parameters:

keywordstr

The keyword to look up in the list of all keywords in the header

startint, optional

The lower bound for the index

stopint, optional

The upper bound for the index

insert(key, card, useblanks=True, after=False)

Inserts a new keyword+value card into the Header at a given location, similar to list.insert.

Parameters:

keyint, str, or tuple

The index into the list of header keywords before which the new keyword should be inserted, or the name of a keyword before which the new keyword should be inserted. Can also accept a (keyword, index) tuple for inserting around duplicate keywords.

cardstr, tuple

A keyword or a (keyword, value, [comment]) tuple; see Header.append

useblanksbool, optional

If there are blank cards at the end of the Header, replace the first blank card so that the total number of cards in the Header does not increase. Otherwise preserve the number of blank cards.

afterbool, optional

If set to True, insert after the specified index or keyword, rather than before it. Defaults to False.

items()

Like dict.items().

keys()

Like dict.keys()–iterating directly over the Header instance has the same behavior.

pop(*args)

Works like list.pop() if no arguments or an index argument are supplied; otherwise works like dict.pop().

popitem()

Similar to dict.popitem().

remove(keyword, ignore_missing=False, remove_all=False)

Removes the first instance of the given keyword from the header similar to list.remove if the Header object is treated as a list of keywords.

Parameters:

keywordstr

The keyword of which to remove the first instance in the header.

ignore_missingbool, optional

When True, ignores missing keywords. Otherwise, if the keyword is not present in the header a KeyError is raised.

remove_allbool, optional

When True, all instances of keyword will be removed. Otherwise only the first instance of the given keyword is removed.

rename_keyword(oldkeyword, newkeyword, force=False)

Rename a card’s keyword in the header.

Parameters:

oldkeywordstr or int

Old keyword or card index

newkeywordstr

New keyword

forcebool, optional

When True, if the new keyword already exists in the header, force the creation of a duplicate keyword. Otherwise a ValueError is raised.

set(keyword, value=None, comment=None, before=None, after=None)

Set the value and/or comment and/or position of a specified keyword.

If the keyword does not already exist in the header, a new keyword is created in the specified position, or appended to the end of the header if no position is specified.

This method is similar to Header.update() prior to Astropy v0.1.

Note

It should be noted that header.set(keyword, value) and header.set(keyword, value, comment) are equivalent to header[keyword] = value and header[keyword] = (value, comment) respectively.

New keywords can also be inserted relative to existing keywords using, for example:

>>> header.insert('NAXIS1', ('NAXIS', 2, 'Number of axes'))

to insert before an existing keyword, or:

>>> header.insert('NAXIS', ('NAXIS1', 4096), after=True)

to insert after an existing keyword.

The only advantage of using Header.set() is that it easily replaces the old usage of Header.update() both conceptually and in terms of function signature.

Parameters:

keywordstr

A header keyword

valuestr, optional

The value to set for the given keyword; if None the existing value is kept, but ‘’ may be used to set a blank value

commentstr, optional

The comment to set for the given keyword; if None the existing comment is kept, but '' may be used to set a blank comment

beforestr, int, optional

Name of the keyword, or index of the Card before which this card should be located in the header. The argument before takes precedence over after if both specified.

afterstr, int, optional

Name of the keyword, or index of the Card after which this card should be located in the header.

setdefault(key, default=None)

Similar to dict.setdefault().

strip()

Strip cards specific to a certain kind of header.

Strip cards like SIMPLE, BITPIX, etc. so the rest of the header can be used to reconstruct another kind of header.

tofile(fileobj, sep='', endcard=True, padding=True, overwrite=False)

Writes the header to file or file-like object.

By default this writes the header exactly as it would be written to a FITS file, with the END card included and padding to the next multiple of 2880 bytes. However, aspects of this may be controlled.

Parameters:

fileobjpath-like object or file-like object, optional

Either the pathname of a file, or an open file handle or file-like object.

sepstr, optional

The character or string with which to separate cards. By default there is no separator, but one could use '\\n', for example, to separate each card with a new line

endcardbool, optional

If True (default) adds the END card to the end of the header string

paddingbool, optional

If True (default) pads the string with spaces out to the next multiple of 2880 characters

overwritebool, optional

If True, overwrite the output file if it exists. Raises an OSError if False and the output file exists. Default is False.

tostring(sep='', endcard=True, padding=True)

Returns a string representation of the header.

By default this uses no separator between cards, adds the END card, and pads the string with spaces to the next multiple of 2880 bytes. That is, it returns the header exactly as it would appear in a FITS file.

Parameters:

sepstr, optional

The character or string with which to separate cards. By default there is no separator, but one could use '\\n', for example, to separate each card with a new line

endcardbool, optional

If True (default) adds the END card to the end of the header string

paddingbool, optional

If True (default) pads the string with spaces out to the next multiple of 2880 characters

Returns:

str

A string representing a FITS header.

totextfile(fileobj, endcard=False, overwrite=False)

Write the header as text to a file or a file-like object.

Equivalent to:

>>> Header.tofile(fileobj, sep='\n', endcard=False,
...               padding=False, overwrite=overwrite)

See also

tofile

update(*args, **kwargs)

Update the Header with new keyword values, updating the values of existing keywords and appending new keywords otherwise; similar to dict.update.

update accepts either a dict-like object or an iterable. In the former case the keys must be header keywords and the values may be either scalar values or (value, comment) tuples. In the case of an iterable the items must be (keyword, value) tuples or (keyword, value, comment) tuples.

Arbitrary arguments are also accepted, in which case the update() is called again with the kwargs dict as its only argument. That is,

>>> header.update(NAXIS1=100, NAXIS2=100)

is equivalent to:

header.update({'NAXIS1': 100, 'NAXIS2': 100})

values()

Like dict.values().