Module pyrap.quanta

Python bindings for casacore Quantum objects It transparently handles Quantity and Quantum<Vector<Double> >.

Introduction

A quantity is a value with a unit. For example, ‘5km/s’, or ‘20Jy/pc2’. This module enables you to create and manipulate such quantities. The types of functionality provided are:

• Conversion of quantities to different units
• Calculations with quantities

Constants, time and angle formatting

If you would like to see all the possible constants known to quanta you can execute the function pyrap.quanta.constants.keys(). You can get the value of any constant in that dictionary with a command such as:

>>> from pyrap import quanta
>>> boltzmann = quanta.constants['k']
>>> print 'Boltzmann constant is ', boltzmann
Boltzmann constant is 1.3806578e-23 J/K

There are some extra handy ways you can manipulate strings when you are dealing with times or angles. The following list shows special strings and string formats which you can input to the quantity function. Something in square brackets is optional. There are examples after the list.

• time: [+-]hh:mm:ss.t... - This is the preferred time format (trailing

  fields can be omitted)

• time: [+-]hhHmmMss.t..[S] - This is an alternative time format (HMS case

  insensitive, trailing second fields can be omitted)

• angle: [+-]dd.mm.ss.t.. - This is the preferred angle format (trailing

  fields after second priod can be omitted; dd.. is valid)

• angle: [+-]ddDmmMss.t...[S] - This is an alternative angle format (DMS

  case insensitive, trailing fields can be omitted after M)

• today - The special string “today” gives the UTC time at the instant

  the command was issued.

• today/time - The special string “today” plus the specified time

  string gives the UTC time at the specified instant

• yyyy/mm/dd[/time] - gives the UTC time at the specified instant

• yyyy-mm-dd[Ttime[+-hh[:mm]]] - gives the UTC time from ISO 8601 format

  with timezone offset

• dd[-]mmm[-][cc]yy[/time] - gives the UTC time at the specified instant

  in calendat style notation (23-jun-1999)

Note that the standard unit for degrees is ‘deg’, and for days ‘d’. Formatting is done in such a way that it interprets a ‘d’ as degrees if preceded by a value without a period and if any value following it is terminated with an ‘m’. In other cases ‘days’ are assumed. Here are some examples:

>>> from pyrap.quanta import quantity
>>> print quantity('today')
50611.2108 d
>>> print quantity('5jul1998')
50999 unit=d
print quantity('5jul1998/12:')
50999.5 d
>>> print quantity('-30.12.2')
30.2005556 deg
>>> print quantity('2:2:10')
30.5416667 deg
>>> print quantity('23h3m2.2s')
345.759167 deg

Python datetime to quantity:

>>> import datetime
>>> utcnow = datetime.datetime.utcnow()
>>> q = quantity(utcnow.isoformat())

The (string) output of quantities can be controlled in different ways:

Standard output:

>>> q = quantity('23h3m2.2s')
>>> print q
345.75917 deg

Angel/time quantity formatting:

>>> print q.formatted("ANGLE")
+345.45.33

Precision formatting:

>>> print q.to_string("%0.2f")
345.76 deg

API

pyrap.quanta.is_quantity(q)

Parameter:q – the object to check.

pyrap.quanta.quantity(*args)

A Factory function to create a pyrap.quanta.Quantity instance. This can be from a scalar or vector and a unit.

Parameter:args –

• A string will be parsed into a pyrap.quanta.Quantity
• A dict with the keys value and unit
• two arguments representing value and unit

Examples:

q1 = quantity(1.0, "km/s")
q2 = quantity("1km/s")
q3 = quantity([1.0,2.0], "km/s")

class pyrap.quanta.Quantity

A unit-value based physical quantity.

set_value(val)

Set the value of the quantity

Parameter:val – The new value to change to (in current units)

get(unit=None)

Return the quantity as another (conformant) one.

Parameter:unit – an optional conformant unit to convert the quantity to. If the unit isn’t specified the canonical unit is used. Return type:pyrap.quanta.Quantity

Example:

>>> q = quantity('1km/s')
>>> print q.get('m/s')
1000.0 m/s

get_value(unit)

Get the value of the quantity suing the optiona unit

Parameter:unit – a conformant unit to convert the quantity to. Return type:float ot list of float

Example:

>>> q = quantity('1km/s')
>>> print q.get_value()
1.0

get_unit()

Retrieve the unit

Return type:string

conforms(other)

Check if another pyrap.quanta.Quantity conforms to self.

Parameter:other – an pyrap.quanta.Quantity object to compare to

convert(other=None)

Convert the quantity using the given Quantity or unit string.

Parameter:other – an optional conformant Quantity to convert to. If other isn’t specified the canonical unit is used.

Example:

>>> q = quantity('1km/s')
>>> q.convert()
>>> print q
1000.0 m/s

to_dict()

Return self as a python dict with value and unit keys.

Return type:dict

to_angle()

Convert to an angle Quantity. This will only work if it conforms to angle

Return type:pyrap.quanta.Quantity

to_time()

Convert to a time Quantity (e.g. hour angle). This will only work if it conforms to time

Return type:pyrap.quanta.Quantity

to_unix_time()

Convert to a unix time value (in seconds). This can be used to create python datetime.datetime objects

Return type:float

to_string(fmt="%0.5f")

Return a string with the Quantity values’ precision formatted with fmt.

Parameter:fmt – the printf type formatting string. Return type:string

formatted(fmt)

Return a formatted string representation of the Quantity.

Parameter:fmt – the format code for angle or time formatting as per casacore angle format and casacore time format Return type:string

On top of the listed method, it also supports all mathematical operators and functions like:

• *, *=, +, +=, -, -=, /, /=
• <, <=, >, >=, ==, !=
• abs, pow, root, srqt, cels, floor, sin, cos, asin, acos, atan, atan2 log, log10, exp
• near and nearabs

Examples:

>>> q = quantity("1km/s")
>>> print q*2
2.0 km/s
>>> print 2*q
2.0 km/s
>>> q /= 2
>>> print q
0.5 km/s
>>> q2 = quantity("0rad")
>>> print dq.cos(q)
1.0