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# This file is part of astro_metadata_translator. 

# 

# Developed for the LSST Data Management System. 

# This product includes software developed by the LSST Project 

# (http://www.lsst.org). 

# See the COPYRIGHT file at the top-level directory of this distribution 

# for details of code ownership. 

# 

# This program is free software: you can redistribute it and/or modify 

# it under the terms of the GNU General Public License as published by 

# the Free Software Foundation, either version 3 of the License, or 

# (at your option) any later version. 

# 

# This program is distributed in the hope that it will be useful, 

# but WITHOUT ANY WARRANTY; without even the implied warranty of 

# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the 

# GNU General Public License for more details. 

# 

# You should have received a copy of the GNU General Public License 

# along with this program. If not, see <http://www.gnu.org/licenses/>. 

 

"""Code to support header manipulation operations.""" 

 

__all__ = ("merge_headers",) 

 

import itertools 

import copy 

 

 

def merge_headers(headers, mode="overwrite"): 

"""Merge multiple headers into a single dict. 

 

Given a list of dict-like data headers, combine them following the 

specified mode. 

 

Parameters 

---------- 

headers : `list` of `dict` (or `dict`-like) 

Collection of headers to combine. `~lsst.daf.base.PropertyList` 

is supported. 

mode : `str` 

Scheme to use when a header has the same key as another header 

but different value. Options are: 

 

- ``'overwrite'`` : Value in later header overwrites earlier value. 

- ``'drop'`` : Entire key is dropped. 

- ``'first'`` : Retain first value encountered. 

- ``'append'`` : Convert value to list with a value for each header 

(`None` if the key was not present). If the value is 

identical in multiple headers but key is missing in 

some, then the single identical header is stored. 

 

Returns 

------- 

merged : `dict` 

Single `dict` combining all the headers using the specified 

combination mode. 

""" 

if not headers: 

raise ValueError("No headers supplied.") 

 

# Force PropertyList to OrderedDict 

# In python 3.7 dicts are guaranteed to retain order 

headers = [h.toOrderedDict() if hasattr(h, "toOrderedDict") else h for h in headers] 

 

if mode == "overwrite": 

merged = copy.deepcopy(headers.pop(0)) 

for h in headers: 

merged.update(h) 

 

elif mode == "first": 

# Reversing the headers and using overwrite mode would result in the 

# header order being inconsistent dependent on mode. 

merged = copy.deepcopy(headers.pop(0)) 

for hdr in headers: 

for key in hdr: 

if key not in merged: 

merged[key] = hdr[key] 

 

elif mode == "drop": 

merged = copy.deepcopy(headers.pop(0)) 

drop = set() 

for hdr in headers: 

for key in hdr: 

if key not in merged: 

merged[key] = hdr[key] 

elif merged[key] != hdr[key]: 

# Key should be dropped later (not in loop since removing 

# the key now might add it back for the next header). 

drop.add(key) 

 

for key in drop: 

del merged[key] 

 

elif mode == "append": 

first = headers.pop(0) 

merged = copy.deepcopy(first) 

fill = set() 

for hdr in headers: 

for key in hdr: 

if key not in merged: 

merged[key] = hdr[key] 

elif not isinstance(merged[key], list) and merged[key] != hdr[key]: 

# If we detect different values, store an empty list 

# in the slot and fill it later. Do it at end so 

# we can pick up earlier values and fill empty with None. 

merged[key] = [] 

fill.add(key) 

 

# Fill the entries that have multiple differing values 

for key in fill: 

merged[key] = [h[key] if key in h else None 

for h in itertools.chain([first], headers)] 

 

else: 

raise ValueError(f"Unsupported value of '{mode}' for mode parameter.") 

 

return merged