2.3 Built-in Functions

The Python interpreter has a number of functions built into it that are always available. They are listed here in alphabetical order.

abs(x)

Return the absolute value of a number. The argument may be a plain or long integer or a floating point number.

apply(function, args[, keywords])

The function argument must be a callable object (a user-defined or built-in function or method, or a class object) and the args argument must be a tuple. The function is called with args as argument list; the number of arguments is the the length of the tuple. (This is different from just calling func(args), since in that case there is always exactly one argument.) If the optional keywords argument is present, it must be a dictionary whose keys are strings. It specifies keyword arguments to be added to the end of the the argument list.

chr(i)

Return a string of one character whose ASCII code is the integer i, e.g., chr(97) returns the string 'a'. This is the inverse of ord(). The argument must be in the range [0..255], inclusive.

cmp(x, y)

Compare the two objects x and y and return an integer according to the outcome. The return value is negative if x < y, zero if x == y and strictly positive if x > y.

coerce(x, y)

Return a tuple consisting of the two numeric arguments converted to a common type, using the same rules as used by arithmetic operations.

compile(string, filename, kind)

Compile the string into a code object. Code objects can be executed by an exec statement or evaluated by a call to eval(). The filename argument should give the file from which the code was read; pass e.g. '<string>' if it wasn't read from a file. The kind argument specifies what kind of code must be compiled; it can be 'exec' if string consists of a sequence of statements, 'eval' if it consists of a single expression, or 'single' if it consists of a single interactive statement (in the latter case, expression statements that evaluate to something else than None will printed).

delattr(object, name)

This is a relative of setattr. The arguments are an object and a string. The string must be the name of one of the object's attributes. The function deletes the named attribute, provided the object allows it. For example, delattr(x, 'foobar') is equivalent to del x.foobar.

dir()

Without arguments, return the list of names in the current local symbol table. With a module, class or class instance object as argument (or anything else that has a __dict__ attribute), returns the list of names in that object's attribute dictionary. The resulting list is sorted. For example:

>>> import sys
>>> dir()
['sys']
>>> dir(sys)
['argv', 'exit', 'modules', 'path', 'stderr', 'stdin', 'stdout']
>>>

divmod(a, b)

Take two numbers as arguments and return a pair of integers consisting of their integer quotient and remainder. With mixed operand types, the rules for binary arithmetic operators apply. For plain and long integers, the result is the same as (a / b, a % b). For floating point numbers the result is the same as (math.floor(a / b), a % b).

eval(expression[, globals[, locals]])

The arguments are a string and two optional dictionaries. The expression argument is parsed and evaluated as a Python expression (technically speaking, a condition list) using the globals and locals dictionaries as global and local name space. If the locals dictionary is omitted it defaults to the globals dictionary. If both dictionaries are omitted, the expression is executed in the environment where eval is called. The return value is the result of the evaluated expression. Syntax errors are reported as exceptions. Example:

>>> x = 1
>>> print eval('x+1')
2
>>>

This function can also be used to execute arbitrary code objects (e.g. created by compile()). In this case pass a code object instead of a string. The code object must have been compiled passing 'eval' to the kind argument.

Hints: dynamic execution of statements is supported by the exec statement. Execution of statements from a file is supported by the execfile() function. The globals() and locals() functions returns the current global and local dictionary, respectively, which may be useful to pass around for use by eval() or execfile().

execfile(file[, globals[, locals]])

This function is similar to the exec statement, but parses a file instead of a string. It is different from the import statement in that it does not use the module administration -- it reads the file unconditionally and does not create a new module.

The arguments are a file name and two optional dictionaries. The file is parsed and evaluated as a sequence of Python statements (similarly to a module) using the globals and locals dictionaries as global and local name space. If the locals dictionary is omitted it defaults to the globals dictionary. If both dictionaries are omitted, the expression is executed in the environment where execfile() is called. The return value is None.

filter(function, list)

Construct a list from those elements of list for which function returns true. If list is a string or a tuple, the result also has that type; otherwise it is always a list. If function is None, the identity function is assumed, i.e. all elements of list that are false (zero or empty) are removed.

float(x)

Convert a number to floating point. The argument may be a plain or long integer or a floating point number.

getattr(object, name)

The arguments are an object and a string. The string must be the name of one of the object's attributes. The result is the value of that attribute. For example, getattr(x, 'foobar') is equivalent to x.foobar.

globals()

Return a dictionary representing the current global symbol table. This is always the dictionary of the current module (inside a function or method, this is the module where it is defined, not the module from which it is called).

hasattr(object, name)

The arguments are an object and a string. The result is 1 if the string is the name of one of the object's attributes, 0 if not. (This is implemented by calling getattr(object, name) and seeing whether it raises an exception or not.)

hash(object)

Return the hash value of the object (if it has one). Hash values are 32-bit integers. They are used to quickly compare dictionary keys during a dictionary lookup. Numeric values that compare equal have the same hash value (even if they are of different types, e.g. 1 and 1.0).

hex(x)

Convert an integer number (of any size) to a hexadecimal string. The result is a valid Python expression.

id(object)

Return the `identity' of an object. This is an integer which is guaranteed to be unique and constant for this object during its lifetime. (Two objects whose lifetimes are disjunct may have the same id() value.) (Implementation note: this is the address of the object.)

input([prompt])

Almost equivalent to eval(raw_input(prompt)). Like raw_input(), the prompt argument is optional. The difference is that a long input expression may be broken over multiple lines using the backslash convention.

int(x)

Convert a number to a plain integer. The argument may be a plain or long integer or a floating point number. Conversion of floating point numbers to integers is defined by the C semantics; normally the conversion truncates towards zero.

len(s)

Return the length (the number of items) of an object. The argument may be a sequence (string, tuple or list) or a mapping (dictionary).

locals()

Return a dictionary representing the current local symbol table. Inside a function, modifying this dictionary does not always have the desired effect.

long(x)

Convert a number to a long integer. The argument may be a plain or long integer or a floating point number.

map(function, list, ...)

Apply function to every item of list and return a list of the results. If additional list arguments are passed, function must take that many arguments and is applied to the items of all lists in parallel; if a list is shorter than another it is assumed to be extended with None items. If function is None, the identity function is assumed; if there are multiple list arguments, map returns a list consisting of tuples containing the corresponding items from all lists (i.e. a kind of transpose operation). The list arguments may be any kind of sequence; the result is always a list.

max(s)

Return the largest item of a non-empty sequence (string, tuple or list).

min(s)

Return the smallest item of a non-empty sequence (string, tuple or list).

oct(x)

Convert an integer number (of any size) to an octal string. The result is a valid Python expression.

open(filename[, mode[, bufsize]])

Return a new file object (described earlier under Built-in Types). The first two arguments are the same as for stdio's fopen(): filename is the file name to be opened, mode indicates how the file is to be opened: 'r' for reading, 'w' for writing (truncating an existing file), and 'a' opens it for appending (which on some Unix systems means that all writes append to the end of the file, regardless of the current seek position). Modes 'r+', 'w+' and 'a+' open the file for updating, provided the underlying stdio library understands this. On systems that differentiate between binary and text files, 'b' appended to the mode opens the file in binary mode. If the file cannot be opened, IOError is raised. If mode is omitted, it defaults to 'r'. The optional bufsize argument specifies the file's desired buffer size: 0 means unbuffered, 1 means line buffered, any other positive value means use a buffer of (approximately) that size. A negative bufsize means to use the system default, which is usually line buffered for for tty devices and fully buffered for other files.

ord(c)

Return the ASCII value of a string of one character. E.g., ord('a') returns the integer 97. This is the inverse of chr().

pow(x, y[, z])

Return x to the power y; if z is present, return x to the power y, modulo z (computed more efficiently than pow(x, y) % z). The arguments must have numeric types. With mixed operand types, the rules for binary arithmetic operators apply. The effective operand type is also the type of the result; if the result is not expressible in this type, the function raises an exception; e.g., pow(2, -1) or pow(2, 35000) is not allowed.

range([start,] end[, step])

This is a versatile function to create lists containing arithmetic progressions. It is most often used in for loops. The arguments must be plain integers. If the step argument is omitted, it defaults to 1. If the start argument is omitted, it defaults to 0. The full form returns a list of plain integers [start, start + step, start + 2 * step, ...]. If step is positive, the last element is the largest start + i * step less than end; if step is negative, the last element is the largest start + i * step greater than end. step must not be zero (or else an exception is raised). Example:

>>> range(10)
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> range(1, 11)
[1, 2, 3, 4, 5, 6, 7, 8, 9, 10]
>>> range(0, 30, 5)
[0, 5, 10, 15, 20, 25]
>>> range(0, 10, 3)
[0, 3, 6, 9]
>>> range(0, -10, -1)
[0, -1, -2, -3, -4, -5, -6, -7, -8, -9]
>>> range(0)
[]
>>> range(1, 0)
[]
>>>

raw_input([prompt])

If the prompt argument is present, it is written to standard output without a trailing newline. The function then reads a line from input, converts it to a string (stripping a trailing newline), and returns that. When EOF is read, EOFError is raised. Example:

>>> s = raw_input('--> ')
--> Monty Python's Flying Circus
>>> s
"Monty Python's Flying Circus"
>>>

reduce(function, list[, initializer])

Apply the binary function to the items of list so as to reduce the list to a single value. E.g., reduce(lambda x, y: x*y, list, 1) returns the product of the elements of list. The optional initializer can be thought of as being prepended to list so as to allow reduction of an empty list. The list arguments may be any kind of sequence.

reload(module)

Re-parse and re-initialize an already imported module. The argument must be a module object, so it must have been successfully imported before. This is useful if you have edited the module source file using an external editor and want to try out the new version without leaving the Python interpreter. The return value is the module object (i.e. the same as the module argument).

There are a number of caveats:

If a module is syntactically correct but its initialization fails, the first import statement for it does not bind its name locally, but does store a (partially initialized) module object in sys.modules. To reload the module you must first import it again (this will bind the name to the partially initialized module object) before you can reload() it.

When a module is reloaded, its dictionary (containing the module's global variables) is retained. Redefinitions of names will override the old definitions, so this is generally not a problem. If the new version of a module does not define a name that was defined by the old version, the old definition remains. This feature can be used to the module's advantage if it maintains a global table or cache of objects -- with a try statement it can test for the table's presence and skip its initialization if desired.

It is legal though generally not very useful to reload built-in or dynamically loaded modules, except for sys, __main__ and __builtin__. In certain cases, however, extension modules are not designed to be initialized more than once, and may fail in arbitrary ways when reloaded.

If a module imports objects from another module using from ... import ..., calling reload() for the other module does not redefine the objects imported from it -- one way around this is to re-execute the from statement, another is to use import and qualified names (module.name) instead.

If a module instantiates instances of a class, reloading the module that defines the class does not affect the method definitions of the instances -- they continue to use the old class definition. The same is true for derived classes.

repr(object)

Return a string containing a printable representation of an object. This is the same value yielded by conversions (reverse quotes). It is sometimes useful to be able to access this operation as an ordinary function. For many types, this function makes an attempt to return a string that would yield an object with the same value when passed to eval().

round(x, n)

Return the floating point value x rounded to n digits after the decimal point. If n is omitted, it defaults to zero. The result is a floating point number. Values are rounded to the closest multiple of 10 to the power minus n; if two multiples are equally close, rounding is done away from 0 (so e.g. round(0.5) is 1.0 and round(-0.5) is -1.0).

setattr(object, name, value)

This is the counterpart of getattr. The arguments are an object, a string and an arbitrary value. The string must be the name of one of the object's attributes. The function assigns the value to the attribute, provided the object allows it. For example, setattr(x, 'foobar', 123) is equivalent to x.foobar = 123.

str(object)

Return a string containing a nicely printable representation of an object. For strings, this returns the string itself. The difference with repr(object) is that str(object) does not always attempt to return a string that is acceptable to eval(); its goal is to return a printable string.

tuple(sequence)

Return a tuple whose items are the same and in the same order as sequence's items. If sequence is alread a tuple, it is returned unchanged. For instance, tuple('abc') returns returns ('a', 'b', 'c') and tuple([1, 2, 3]) returns (1, 2, 3).

type(object)

Return the type of an object. The return value is a type object. The standard module types defines names for all built-in types.

   For instance:

>>> import types
>>> if type(x) == types.StringType: print "It's a string"

vars([object])

Without arguments, return a dictionary corresponding to the current local symbol table. With a module, class or class instance object as argument (or anything else that has a __dict__ attribute), returns a dictionary corresponding to the object's symbol table. The returned dictionary should not be modified: the effects on the corresponding symbol table are undefined.

xrange([start,] end[, step])

This function is very similar to range(), but returns an ``xrange object'' instead of a list. This is an opaque sequence type which yields the same values as the corresponding list, without actually storing them all simultaneously. The advantage of xrange() over range() is minimal (since xrange() still has to create the values when asked for them) except when a very large range is used on a memory-starved machine (e.g. MS-DOS) or when all of the range's elements are never used (e.g. when the loop is usually terminated with break).