Headers

Header

class pyfits.Header(cards=, []txtfile=None)

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 PyFITS documentation for more details on working with headers.

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

Parameters:  

cards : A list of Card objects, optional

The cards to initialize the header with.

txtfile : file path, file object or file-like object, optional

Input ASCII header parameters file (Deprecated) Use the Header.fromfile classmethod instead.

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

Add a blank card.

Parameters:  

value : str, optional

Text to be added.

before : str or int, optional

Same as in Header.update

after : str or int, optional

Same as in Header.update

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

Add a COMMENT card.

Parameters:  

value : str

Text to be added.

before : str or int, optional

Same as in Header.update

after : str or int, optional

Same as in Header.update

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

Add a HISTORY card.

Parameters:  

value : str

History text to be added.

before : str or int, optional

Same as in Header.update

after : str 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:  

card : str, 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

useblanks : bool, 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.

bottom : bool, optional

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

end : bool, optional

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

clear()

Remove all cards from the header.

copy(strip=False)

Make a copy of the Header.

Parameters:  

strip : bool, 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:  

keyword : str

The keyword to count instances of in the header

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:  

cards : iterable

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

strip : bool, 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).

unique : bool, 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.

update : bool, optional

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

update_first : bool, 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, end : bool, optional

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

fromTxtFile(*args, **kwargs)

Deprecated since version 3.1: This is equivalent to self.extend(Header.fromtextfile(fileobj), update=True, update_first=True). Note that there there is no direct equivalent to the replace=True option since Header.fromtextfile() returns a new Header instance.

Input the header parameters from an ASCII file.

The input header cards will be used to update the current header. Therefore, when an input card key matches a card key that already exists in the header, that card will be updated in place. Any input cards that do not already exist in the header will be added. Cards will not be deleted from the header.

Parameters:  

fileobj : file path, file object or file-like object

Input header parameters file.

replace : bool, optional

When True, indicates that the entire header should be replaced with the contents of the ASCII file instead of just updating the current 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:  

fileobj : str, file-like

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.

sep : str, 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).

endcard : bool, 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 IOError is raised.

padding : bool, 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.

value : optional

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:  

data : str

String containing the entire header.

sep : str, 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).

Returns:  

header :

A new Header instance.

classmethod fromtextfile(fileobj, endcard=False)

Equivalent to:

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

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:  

key : str

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

default : optional

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

Returns:  

value :

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

get_comment(*args, **kwargs)

Deprecated since version 3.1: Use header['COMMENT'] instead.

Get all comment cards as a list of string texts.

get_history(*args, **kwargs)

Deprecated since version 3.1: Use header['HISTORY'] instead.

Get all history cards as a list of string texts.

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:  

keyword : str

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

start : int, optional

The lower bound for the index

stop : int, 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:  

key : int, str, or tuple

The index into the 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.

card : str, tuple

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

useblanks : bool, 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.

after : bool, optional

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

items()

Like dict.items().

iteritems()

Like dict.iteritems().

iterkeys()

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

itervalues()

Like dict.itervalues().

keys()

Return a list of keywords in the header in the order they appear–like dict.keys() but ordered.

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)

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:  

value : str

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

rename_key(*args, **kwargs)

Deprecated since version 3.1: Use Header.rename_keyword() instead.

rename_keyword(oldkeyword, newkeyword, force=False)

Rename a card’s keyword in the header.

Parameters:  

oldkeyword : str or int

Old keyword or card index

newkeyword : str

New keyword

force : bool, 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 PyFITS 3.1.

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) respectfully.

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 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:  

keyword : str

A header keyword

value : str, 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

comment : str, 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

before : str, 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.

after : str, 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.setitem().

toTxtFile(*args, **kwargs)

Deprecated since version 3.1: Use Header.totextfile() instead.

Output the header parameters to a file in ASCII format.

Parameters:  

fileobj : file path, file object or file-like object

Output header parameters file.

clobber : bool

When True, overwrite the output file if it exists.

tofile(fileobj, sep='', endcard=True, padding=True, clobber=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:  

fileobj : str, file, optional

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

sep : str, 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

endcard : bool, optional

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

padding : bool, optional

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

clobber : bool, optional

If True, overwrites the output file if it already exists

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:  

sep : str, 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

endcard : bool, optional

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

padding : bool, optional

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

Returns:  

s : string

A string representing a FITS header.

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

Equivalent to:

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

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})

Warning

As this method works similarly to dict.update it is very different from the Header.update() method in PyFITS versions prior to 3.1.0. However, support for the old API is also maintained for backwards compatibility. If update() is called with at least two positional arguments then it can be assumed that the old API is being used. Use of the old API should be considered deprecated. Most uses of the old API can be replaced as follows:

• Replace

  >>> header.update(keyword, value)
  

  with

  >>> header[keyword] = value
  

• Replace

  >>> header.update(keyword, value, comment=comment)
  

  with

  >>> header[keyword] = (value, comment)
  

• Replace

  >>> header.update(keyword, value, before=before_keyword)
  

  with

  >>> header.insert(before_keyword, (keyword, value))
  

• Replace

  >>> header.update(keyword, value, after=after_keyword)
  

  with

  >>> header.insert(after_keyword, (keyword, value),
  ...               after=True)
  

See also Header.set() which is a new method that provides an interface similar to the old Header.update() and may help make transition a little easier.

For reference, the old documentation for the old Header.update() is provided below:

Update one header card.

If the keyword already exists, it’s value and/or comment will be updated. If it does not exist, a new card will be created and it will be placed before or after the specified location. If no before or after is specified, it will be appended at the end.

Parameters:  

key : str

keyword

value : str

value to be used for updating

comment : str, optional

to be used for updating, default=None.

before : str, int, optional

name of the keyword, or index of the Card before which the new card will be placed. The argument before takes precedence over after if both specified.

after : str, int, optional

name of the keyword, or index of the Card after which the new card will be placed.

savecomment : bool, optional

When True, preserve the current comment for an existing keyword. The argument savecomment takes precedence over comment if both specified. If comment is not specified then the current comment will automatically be preserved.

values()

Returns a list of the values of all cards in the header.

ascard

Deprecated since version 3.1: Use the cards attribute instead.

Returns a CardList object wrapping this Header; provided for backwards compatibility for the old API (where Headers had an underlying CardList).

cards

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

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'