Units

Casting rules

In Brian 1, a distinction is made between scalars and numpy arrays (including scalar arrays): Scalars could be multiplied with a unit, resulting in a Quantity object whereas the multiplication of an array with a unit resulted in a (unitless) array. Accordingly, scalars where considered as dimensionless quantities for the purpose of unit checking (e.g.. 1 + 1 * mV raised an error) whereas arrays where not (e.g. array(1) + 1 * mV resulted in 1.001 without any errors). Brian 2 no longer makes this distinction and treats both scalars and arrays as dimensionless for unit checking and make all operations involving quantities return a quantity.:

>> 1 + 1 * second
DimensionMismatchError: Addition, dimensions were (s) (1)

>> np.array([1]) + 1 * second
DimensionMismatchError: Addition, dimensions were (s) (1)

>> 1 * second  + 1 * second
2.0 * second

>> np.array([1]) * second  + 1 * second
array([ 2.]) * second

As one exception from this rule, a scalar or array 0 is considered as having “any unit”, i.e. 0 + 1 * second will result in 1 * second without a dimension mismatch error and 0 == 0 * mV will evaluate to True. This seems reasonable from a mathematical viewpoint and makes some sources of error disappear. For example, the Python builtin sum (not numpy’s version) adds the value of the optional argument start, which defaults to 0, to its main argument. Without this exception, sum([1 * mV, 2 * mV]) would therefore raise an error.

The above rules also apply to all comparisons (e.g. == or <) with one further exception: inf and -inf also have “any unit”, therefore an expression like v <= inf will never raise an exception (and always return True).

Functions and units

ndarray methods

All methods that make sense on quantities should work, i.e. they check for the correct units of their arguments and return quantities with units were appropriate. Most of the methods are overwritten using thin function wrappers:

wrap_function_keep_dimension:
Strips away the units before giving the array to the method of ndarray, then reattaches the unit to the result (examples: sum, mean, max)
wrap_function_change_dimension:
Changes the dimensions in a simple way that is independent of function arguments, the shape of the array, etc. (examples: sqrt, var, power)
wrap_function_dimensionless:
Raises an error if the method is called on a quantity with dimensions (i.e. it works on dimensionless quantities).

List of methods

all, any, argmax, argmax, argsort, clip, compress, conj, conjugate, copy, cumsum, diagonal, dot, dump, dumps, fill, flatten, getfield, item, itemset, max, mean, min, newbyteorder, nonzero, prod, ptp, put, ravel, repeat, reshape, round, searchsorted, setasflat, setfield, setflags, sort, squeeze, std, sum, take, tolist, trace, transpose, var, view

Notes

  • Methods directly working on the internal data buffer (setfield, getfield, newbyteorder) ignore the dimensions of the quantity.
  • The type of a quantity cannot be int, therefore astype does not quite work when trying to convert the array into integers.
  • choose is only defined for integer arrays and therefore does not work
  • tostring and tofile only return/save the pure array data without the unit (but you can use dump or dumps to pickle a quantity array)
  • resize does not work: ValueError: cannot resize this array: it does not own its data
  • cumprod would result in different dimensions for different elements and is therefore forbidden
  • item returns a pure Python float by definition
  • itemset does not check for units

Numpy ufuncs

All of the standard numpy ufuncs (functions that operate element-wise on numpy arrays) are supported, meaning that they check for correct units and return appropriate arrays. These functions are often called implicitly, for example when using operators like < or **.

Math operations:
add, subtract, multiply, divide, logaddexp, logaddexp2, true_divide, floor_divide, negative, power, remainder, mod, fmod, absolute, rint, sign, conj, conjugate, exp, exp2, log, log2, log10, expm1, log1p, sqrt, square, reciprocal, ones_like
Trigonometric functions:
sin, cos, tan, arcsin, arccos, arctan, arctan2, hypot, sinh, cosh, tanh, arcsinh, arccosh, arctanh, deg2rad, rad2deg
Bitwise functions:
bitwise_and, bitwise_or, bitwise_xor, invert, left_shift, right_shift
Comparison functions:
greater, greater_equal, less, less_equal, not_equal, equal, logical_and, logical_or, logical_xor, logical_not, maximum, minimum
Floating functions:
isreal, iscomplex, isfinite, isinf, isnan, floor, ceil, trunc, fmod

Not taken care of yet: signbit, copysign, nextafter, modf, ldexp, frexp

Notes

  • Everything involving log or exp, as well as trigonometric functions only works on dimensionless array (for arctan2 and hypot this is questionable, though)
  • Unit arrays can only be raised to a scalar power, not to an array of exponents as this would lead to differing dimensions across entries. For simplicity, this is enforced even for dimensionless quantities.
  • Bitwise functions never works on quantities (numpy will by itself throw a TypeError because they are floats not integers).
  • All comparisons only work for matching dimensions (with the exception of always allowing comparisons to 0) and return a pure boolean array.
  • All logical functions treat quantities as boolean values in the same way as floats are treated as boolean: Any non-zero value is True.

Numpy functions

Many numpy functions are functional versions of ndarray methods (e.g. mean, sum, clip). They therefore work automatically when called on quantities, as numpy propagates the call to the respective method.

There are some functions in numpy that do not propagate their call to the corresponding method (because they use np.asarray instead of np.asanyarray, which might actually be a bug in numpy): trace, diagonal, ravel, dot. For these, wrapped functions in unitsafefunctions.py are provided.

Wrapped numpy functions in unitsafefunctions.py

These functions are thin wrappers around the numpy functions to correctly check for units and return quantities when appropriate:

log, exp, sin, cos, tan, arcsin, arccos, arctan, sinh, cosh, tanh, arcsinh, arccosh, arctanh, diagonal, ravel, trace, dot

numpy functions that work unchanged

This includes all functional counterparts of the methods mentioned above (with the exceptions mentioned above). Some other functions also work correctly, as they are only using functions/methods that work with quantities:

  • linspace, diff, digitize [1]
  • trim_zeros, fliplr, flipud, roll, rot90, shuffle
  • corrcoeff [1]
[1](1, 2) But does not care about the units of its input.

numpy functions that return a pure numpy array instead of quantities

  • arange
  • cov
  • random.permutation
  • histogram, histogram2d
  • cross, inner, outer
  • where

numpy functions that do something wrong

  • insert, delete (return a quantity array but without units)
  • correlate (returns a quantity with wrong units)
  • histogramdd (raises a DimensionMismatchError)

User-defined functions and units

For performance and simplicity reasons, code within the Brian core does not use Quantity objects but unitless numpy arrays instead. See Adding support for new functions for details on how to make use user-defined functions with Brian’s unit system.