Built-In Functions

Built-In Functions
	Chapter 4. Simple Numeric Expressions and Output

Python has a number of built-in functions, which are an integral part of the Python interpreter. We can't look at all of them because many are related to features of the language that we haven't addressed yet.

One of the built-in mathematical functions will have to wait for complete coverage until we've introduced the more complex data types, specifically tuples, in Chapter 13, Tuples. The divmod (x, y ) function returns a tuple object with the quotient and remainder in division.

Built-In Math Functions

The bulk of the math functions are in a separate module, called math, which we will cover in the section called “The math Module”. The formal definitions of mathematical built-in functions are provided below.

abs (number) → number: Return the absolute value of the argument, |x|.
pow (x, y, [z]) → number: Raise x to the y power. If z is present, this is done modulo z, x^y % z.
round (number, [ndigits]) → float: Round number to ndigits beyond the decimal point.
cmp (x, y) → integer: Compare x and y, returning a number. If the number is less than 0, then x<y; if the number is zero, then x == y; if the number is positive, then x>y.

The round (number, [ndigits] ) function rounds a number to the nearest whole number. If the n parameter is given, this is the number of decimal places to round to. If n is positive, this is decimal places to the right of the decimal point. If n is negative, this is the number of places to the left of the decimal point. Examples: round(678.456,2) yields 678.46; round(678.456,-1) yields 680.

The cmp(x, y) function is handy for the comparisons used when sorting objects into order. For example, cmp(2,35) yields -1, telling us that the first value is less than the second value.

String Conversion Functions

The string conversion functions provide alternate representations for numeric values. This list expands on the function definitions in the section called “Numeric Conversion Functions”.

hex (number) → string: Create a hexadecimal string representation of number. A leading '0x' is placed on the string as a reminder that this is hexadecimal. hex(684) yields the string '0x2ac'.
oct (number) → string: Create a octal string representation of number. A leading '0' is placed on the string as a reminder that this is octal not decimal. oct(509) yields the string '0775'.
int (string, [base]) → integer: Generates an integer from the string x. If base is supplied, x must be in the given base. If base is omitted, x must be decimal.
str (object) → string: Generate a string representation of the given object. This is the a "readable" version of the value.
repr (object) → string: Generate a string representation of the given object. Generally, this is the a Python expression that can reconstruct the value; it may be rather long and complex.

The int function has two forms. The int (x) form converts a decimal string, x, to an integer. For example int('25') is 25. The int(x, b) form converts a string, x, in base b to an integer. For example int('010101',2) yields 21. int('321',4) is 57, int('2ac',16) is 684.

The str(x) and repr(x) functions convert any Python object to a string. The str(x) version is typically more readable, where the repr(x) version is an internalized representation. For most garden-variety numeric values, there is no difference. For the more complex data types, however, the resultsof repr and str can be very different. For classes you write (see Chapter 21, Classes), your class definition must provide these string representation functions.

Collection Functions

These are two built-in functions which operate on a collection of data elements.

max (sequence) → value: Return the largest value in sequence.
min (sequence) → value: Return the smallest value in sequence.

The max and min functions accept any number of values and return the largest or smallest of the values. max(1,2,3) yields 3, min(1,2,3) yields 1.