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, optionalThe 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 theunique
argument. Commentary keywords are treated the same as ifunique=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 ifupdate=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 thereplace=True
option sinceHeader.fromtextfile()
returns a newHeader
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 newHeader
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
-
items
()¶ Like
dict.items()
.
-
iteritems
()¶ Like
dict.iteritems()
.
-
iterkeys
()¶ Like
dict.iterkeys()
–iterating directly over theHeader
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 likedict.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 aValueError
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)
andheader.set(keyword, value, comment)
are equivalent toheader[keyword] = value
andheader[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 ofHeader.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 commentbefore : str, int, optional
Name of the keyword, or index of the
Card
before which this card should be located in the header. The argumentbefore
takes precedence overafter
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 lineendcard : bool, optional
If
True
(default) adds the END card to the end of the header stringpadding : bool, optional
If
True
(default) pads the string with spaces out to the next multiple of 2880 charactersclobber : 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 lineendcard : 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 theHeader.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 oldHeader.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
orafter
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 argumentbefore
takes precedence overafter
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 argumentsavecomment
takes precedence overcomment
if both specified. Ifcomment
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 underlyingCardList
).
-
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'
-