Introduction

pdstemplate is a Python module that provide the PdsTemplate class, used to generate PDS labels based on templates. Although specifically designed to facilitate data deliveries by PDS data providers, the template system is generic and could be used to generate files from templates for other purposes.

pdstemplate is a product of the PDS Ring-Moon Systems Node.

Installation

The pdstemplate module is available via the rms-pdstemplate package on PyPI and can be installed with:

pip install rms-template

Getting Started

The general procedure is as follows:

1. Create a template object by calling the PdsTemplate constructor to read a template file:

    template = PdsTemplate(template_file_path)
   

2. Create a dictionary that contains the parameter values to use inside the label.

3. Construct the label as follows:

    template.write(dictionary, label_file)
   

   This will create a new label of the given name, using the values in the given dictionary. Once the template has been constructed, steps 2 and 3 can be repeated any number of times.

Details of the PdsTemplate class are available in the module documentation.

SUBSTITUTIONS

A template file will look generally like a label file, except for certain embedded expressions that will be replaced when the template's write() method is called.

In general, everything between dollar signs $ in the template is interpreted as a Python expression to be evaluated. The result of this expression then replaces it inside the label. For example, if dictionary['INSTRUMENT_ID'] == 'ISSWA', then

<instrument_id>$INSTRUMENT_ID$</instrument_id>

in the template will become

<instrument_id>ISSWA</instrument_id>

in the label. The expression between $ in the template can include indexes, function calls, or just about any other Python expression. As another example, using the same dictionary above,

<camera_fov>$"Narrow" if INSTRUMENT_ID == "ISSNA" else "Wide"$</camera_fov>

in the template will become

<camera_fov>Wide</camera_fov>

in the label.

An expression in the template of the form $name=expression$, where the name is a valid Python variable name, will also also have the side-effect of defining this variable so that it can be re-used later in the template. For example, if this appears as an expression,

$cruise_or_saturn=('cruise' if START_TIME < 2004 else 'saturn')$

then later in the template, one can write:

<lid_reference>
urn:nasa:pds:cassini_iss_$cruise_or_saturn$:data_raw:cum-index
</lid_reference>

To embed a literal $ inside a label, enter $$ into the template.

PRE-DEFINED FUNCTIONS

The following pre-defined functions can be used inside any expression in the template.

• BASENAME(filepath): The basename of filepath, with leading directory path removed.

• BOOL(value, true='true', false='false'): Return true if value evaluates to Boolean True; otherwise, return false.

• COUNTER(name, reset=False): The current value of a counter identified by name, starting at 1. If reset is True, the counter is reset to 0.

• CURRENT_TIME(date_only=False): The current time in the local time zone as a string of the form "yyyy-mm-ddThh:mm:sss" if date_only=False or "yyyy-mm-dd" if date_only=True.

• CURRENT_ZULU(date_only=False): The current UTC time as a string of the form "yyyy-mm-ddThh:mm:sssZ" if date_only=False or "yyyy-mm-dd" if date_only=True.

• DATETIME(time, offset=0, digits=None): Convert time as an arbitrary date/time string or TDB seconds to an ISO date format with a trailing "Z". An optional offset in seconds can be applied. The returned string contains an appropriate number of decimal digits in the seconds field unless digits is specified explicitly. If time is "UNK", then "UNK" is returned.

• DATETIME_DOY(time, offset=0, digits=None): Convert time as an arbitrary date/time string or TDB seconds to an ISO date of the form "yyyy-dddThh:mm:ss[.fff]Z". An optional offset in seconds can be applied. The returned string contains an appropriate number of decimal digits in the seconds field unless digits is specified explicitly. If time is "UNK", then "UNK" is returned.

• DAYSECS(string): The number of elapsed seconds since the most recent midnight. time can be a date/time string, a time string, or TDB seconds.

• FILE_BYTES(filepath): The size in bytes of the file specified by filepath.

• FILE_MD5(filepath): The MD5 checksum of the file specified by filepath.

• FILE_RECORDS(filepath): The number of records in the the file specified by filepath if it is ASCII; 0 if the file is binary.

• FILE_TIME(filepath): The modification time in the local time zone of the file specified by filepath in the form "yyyy-mm-ddThh:mm:ss".

• FILE_ZULU(filepath): The UTC modification time of the the file specified by filepath in the form "yyyy-mm-ddThh:mm:ssZ".

• LABEL_PATH(): The full directory path to the label file being written.

• NOESCAPE(text): If the template is XML, evaluated expressions are "escaped" to ensure that they are suitable for embedding in a PDS4 label. For example, ">" inside a string will be replaced by >. This function prevents text from being escaped in the label, allowing it to contain literal XML.

• RAISE(exception, message): Raise an exception with the given class exception and the message.

• REPLACE_NA(value, if_na, flag='N/A'): Return if_na if value equals "N/A" (or flag if specified); otherwise, return value.

• REPLACE_UNK(value, if_unk): Return if_unk if value equals "UNK"; otherwise, return value.

• TEMPLATE_PATH(): The directory path to the template file.

• VERSION_ID(): Version ID of this module, e.g., "v0.1.0".

• WRAP(left, right, text, preserve_single_newlines=True): Format text to fit between the left and right column numbers. The first line is not indented, so the text will begin in the column where "$WRAP" first appears in the template. If preserve_single_newlines is true, then all newlines in the string will show up in the resultant text. If false, then single newlines will be considered part of the text flow and will be wrapped.

These functions can also be used directly by the programmer; they are static functions of class PdsTemplate.

COMMENTS

Any text appearing on a line after the symbol $NOTE: will not appear in the label. Trailing blanks resulting from this removal are also removed.

HEADERS

The template may also contain any number of headers. These appear alone on a line of the template and begin with $ as the first non-blank character. They determine whether or how subsequent text of the template will appear in the file, from here up to the next header line.

You can include one or more repetitions of the same text using $FOR and $END_FOR headers. The format is

$FOR(expression)
    <template text>
$END_FOR

where expression evaluates to a Python iterable. Within the template text, these new variable names are assigned:

• VALUE = the next value of the iterator;
• INDEX = the index of this iterator, starting at zero;
• LENGTH = the number of items in the iteration.

For example, if

dictionary["targets"] = ["Jupiter", "Io", "Europa"]
dictionary["naif_ids"] = [599, 501, 502],

then

$FOR(targets)
    <target_name>$VALUE (naif_ids[INDEX])$</target_name>
$END_FOR

in the template will become

<target_name>Jupiter (599)</target_name>
<target_name>Io (501)</target_name>
<target_name>Europa (502)</target_name>

in the label.

Instead of using the names VALUE, INDEX, and LENGTH, you can customize the variable names by listing up to three comma-separated names and an equal sign = before the iterable expression. For example, this will produce the same results as the example above:

$FOR(name, k=targets)
    <target_name>$name (naif_ids[k])$</target_name>
$END_FOR

You can also use $IF, $ELSE_IF, $ELSE, and $END_IF headers to select among alternative blocks of text in the template:

• $IF(expression) - Evaluate expression and include the next lines of the template if it is logically True (e.g., boolean True, a nonzero number, a non-empty list or string, etc.).
• $ELSE_IF(expression) - Include the next lines of the template if expression is logically True and every previous expression was not.
• $ELSE - Include the next lines of the template only if all prior expressions were logically False.
• $END_IF - This marks the end of the set of if/else alternatives.

As with other substitutions, you can define a new variable of a specified name by using name=expression inside the parentheses of an $IF() or $ELSE_IF() header.

Note that headers can be nested arbitrarily inside the template.

You can use the $NOTE and $END_NOTE headers to embed any arbitrary comment block into the template. Any text between these headers does not appear in the label.

One additional header is supported: $ONCE(expression). This header evaluates expression but does not alter the handling of subsequent lines of the template. You can use this capability to define variables internally without affecting the content of the label produced. For example:

$ONCE(date=big_dictionary["key"]["date"])

will assign the value of the variable named "date" for subsequent use within the template.

LOGGING AND EXCEPTION HANDLING

The pdslogger module is used to handle logging. By default, the pdslogger.NullLogger class is used, meaning that no actions are logged. To override, call

set_logger(logger)

in your Python program to use the specified logger. For example,

set_logger(pdslogger.EasyLogger())

will log all messages to the terminal.

By default, exceptions during a call to write() or generate() are handled as follows:

1. They are written to the log.
2. The template attribute ERROR_COUNT contains the number of exceptions raised.
3. The expression that triggered the exception is replaced by the error text in the label, surrounded by [[[ and ]]] to make it easier to find.
4. The exception is otherwise suppressed.

This behavior can be modified by calling method raise_exceptions(True). In this case, the call to write() or generate() raises the exception and then halts.