Condition Syntax

Conditions in PyTables are used in methods related with in-kernel and indexed searches such as Table.where() or Table.read_where(). They are interpreted using Numexpr, a powerful package for achieving C-speed computation of array operations (see [NUMEXPR]).

A condition on a table is just a string containing a Python expression involving at least one column, and maybe some constants and external variables, all combined with algebraic operators and functions. The result of a valid condition is always a boolean array of the same length as the table, where the i-th element is true if the value of the expression on the i-th row of the table evaluates to true

That is the reason why multidimensional fields in a table are not supported in conditions, since the truth value of each resulting multidimensional boolean value is not obvious. Usually, a method using a condition will only consider the rows where the boolean result is true.

For instance, the condition ‘sqrt(x*x + y*y) < 1’ applied on a table with x and y columns consisting of floating point numbers results in a boolean array where the i-th element is true if (unsurprisingly) the value of the square root of the sum of squares of x and y is less than 1. The sqrt() function works element-wise, the 1 constant is adequately broadcast to an array of ones of the length of the table for evaluation, and the less than operator makes the result a valid boolean array. A condition like ‘mycolumn’ alone will not usually be valid, unless mycolumn is itself a column of scalar, boolean values.

In the previous conditions, mycolumn, x and y are examples of variables which are associated with columns. Methods supporting conditions do usually provide their own ways of binding variable names to columns and other values. You can read the documentation of Table.where() for more information on that. Also, please note that the names None, True and False, besides the names of functions (see below) can not be overridden, but you can always define other new names for the objects you intend to use.

Values in a condition may have the following types:

  • 8-bit boolean (bool).
  • 32-bit signed integer (int).
  • 64-bit signed integer (long).
  • 32-bit, single-precision floating point number (float or float32).
  • 64-bit, double-precision floating point number (double or float64).
  • 2x64-bit, double-precision complex number (complex).
  • Raw string of bytes (str).

Nevertheless, if the type passed is not among the above ones, it will be silently upcasted, so you don’t need to worry too much about passing supported types, except for the Unsigned 64 bits integer, that cannot be upcasted to any of the supported types.

However, the types in PyTables conditions are somewhat stricter than those of Python. For instance, the only valid constants for booleans are True and False, and they are never automatically cast to integers. The type strengthening also affects the availability of operators and functions. Beyond that, the usual type inference rules apply.

Conditions support the set of operators listed below:

  • Logical operators: &, |, ~.
  • Comparison operators: <, <=, ==, !=, >=, >.
  • Unary arithmetic operators: -.
  • Binary arithmetic operators: +, -, *, /, **, %.

Types do not support all operators. Boolean values only support logical and strict (in)equality comparison operators, while strings only support comparisons, numbers do not work with logical operators, and complex comparisons can only check for strict (in)equality. Unsupported operations (including invalid castings) raise NotImplementedError exceptions.

You may have noticed the special meaning of the usually bitwise operators &, | and ~. Because of the way Python handles the short-circuiting of logical operators and the truth values of their operands, conditions must use the bitwise operator equivalents instead. This is not difficult to remember, but you must be careful because bitwise operators have a higher precedence than logical operators. For instance, ‘a and b == c’ (a is true AND b is equal to c) is not equivalent to ‘a & b == c’ (a AND b is equal to c). The safest way to avoid confusions is to use parentheses around logical operators, like this: ‘a & (b == c)’. Another effect of short-circuiting is that expressions like ‘0 < x < 1’ will not work as expected; you should use ‘(0 < x) & (x < 1)’.

All of this may be solved if Python supported overloadable boolean operators (see PEP 335) or some kind of non-shortcircuiting boolean operators (like C’s &&, || and !).

You can also use the following functions in conditions:

  • where(bool, number1, number2): number - number1 if the bool condition is true, number2 otherwise.
  • {sin,cos,tan}(float|complex): float|complex - trigonometric sine, cosine or tangent.
  • {arcsin,arccos,arctan}(float|complex): float|complex - trigonometric inverse sine, cosine or tangent.
  • arctan2(float1, float2): float - trigonometric inverse tangent of float1/float2.
  • {sinh,cosh,tanh}(float|complex): float|complex - hyperbolic sine, cosine or tangent.
  • {arcsinh,arccosh,arctanh}(float|complex): float|complex - hyperbolic inverse sine, cosine or tangent.
  • {log,log10,log1p}(float|complex): float|complex - natural, base-10 and log(1+x) logarithms.
  • {exp,expm1}(float|complex): float|complex - exponential and exponential minus one.
  • sqrt(float|complex): float|complex - square root.
  • abs(float|complex): float|complex - absolute value.
  • {real,imag}(complex): float - real or imaginary part of complex.
  • complex(float, float): complex - complex from real and imaginary parts.