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A Python Bestiary Itemizing Python Objects and Nuances $Revision: 1.1 $ $Date: 1994/08/05 15:57:31 $ ken.manheimer@nist.gov Notable lexical entities ======================== Keywords access and break class continue def del elif else except exec finally for from global if import in is lambda not or pass print raise return try while String Literal Escapes \newline Ignored (escape newline) \\ Backslash (\) \e Escape (ESC) \v Vertical Tab (VT) \' Single quote (') \f Formfeed (FF) \0XX char with \" Double quote (") \n Linefeed (LF) octal value XX \a Bell (BEL) \r Carriage Return (CR) \xXX char with \b Backspace (BS) \t Horizontal Tab (TAB) hex value XX Illegitimate Tokens (only valid in strings): @ $ ? Operating environment (Unix) Shell environment vars: PYTHONPATH: dirs to be prepended to module search path; a la shell PATH PYTHONSTARTUP: names path of file to be loaded on interactive startup Internal (module sys) vars ('import sys' to refer to them): argv - list of command and arguments passed to python script builtin_module_names - list of names of modules compiled in to python ( exc_* values are set only during handling of a raised exception) exc_type - type of exception being handled exc_value - exception's parameter ('raise's 2nd arg) exc_traceback - exception's traceback obj exit(N) - exit from python with status N, by raising SystemExit exitfunc - function hook, to be called on exit if set last_type - type of last unhandled exception last_value - value of last unhandled exception last_traceback - traceback of last unhandled exception modules - list of modules already loaded path - list of strings specifying module-load search path ps1='>>>' - string specifying primary prompt, iff interactive ps2='...' - string specifying secondary prompt, iff interactive settrace - system trace function, used by debuggers, etc setprofile - system profile function stdin - standard input file object stdout - standard output file object stderr - standard error file object tracebacklimit=1000 - maximum number of traceback levels to print Objects - behaviors, special attributes, operations, statements, etc ==================================================================== General object aspects Boolean values and operators False values: None, numeric zeros, empty sequences and mappings True values: all other values not X: if X is false then 1, else 0 ( 'or', 'and' evaluate second arg only if necessary to determine outcome) X or Y: if X is false then Y, else X X and Y: if X is false then X, else Y Special methods for any type (s: self) id(obj) unique identifier for object (currently, its address) __init__(s, args) object instantiation - see Classes, below __del__(s) object demise __repr__(s) repr() and `...` conversions __str__(s) str() and 'print' statement __cmp__(s) implements <, ==, >, <=, <>, !=, >=, is [not] __hash__(s) hash() and dictionary operations Special informative state attributes for some types: X.__dict__ dict used to store object's writeable attributes X.__methods__ list of X's methods; on many built-in types. X.__members__ lists of X's data attributes X.__class__ class to which X belongs X.__bases__ tuple of X base classes General Name Space behavior and binding Name space search order: local, global, builtin "global" name space = file = module "local" name space = function or method Code Block scopes (ns = name space, cb = containing block) ( global ns generally is containing module, unless overriden by args) Code block type Local ns Notes --------------- -------- ----- Module same as global ns Script same as global ns global ns is __main__ Interactive cmd same as global ns global ns is __main__ Class def new ns Function body new ns 'exec' string local ns of cb (or args) 'eval' string local ns of caller (or args) 'execfile' file local ns of caller (or args) 'input' expr local ns of caller Binding operations and exceptions: ~ SyntaxError: on attempt to bind to literals or other non-bindables ~ NameError: on attempt to evaluate unbound atom ( for object attribute functions, obj: object, nm: string, val: any value) getattr(obj, nm) get value of obj.nm hasattr(obj, nm) true if obj has nm attribute setattr(obj, nm, val) set obj.nm to val assignment statement: targ1, targ2, ,,, = obj1, obj2, ,,, deletion statement: del obj1, obj2, ... for loop target identifiers, 'except' clauses (see Statements, below) formal params (see Callables, below) import statement (see Modules objects, below) class and func defs (see Callables, below) Name space linkage global statement: global var, ... # Interpret 'var's as globals X access statement: access ... # control inst and class vars access @ Built-in Exceptions AttributeError On attribute reference or assignment failure EOFError Immediate end-of-file hit by input() or raw_input() IOError I/O-related I/O operation failure ImportError On failure of `import' to find module or name IndexError On out-of-range sequence subscript KeyError On reference to a non-existent mapping (dict) key KeyboardInterrupt On user entry of the interrupt key (often `Control-C') MemoryError On recoverable memory exhaustion NameError On failure to find a local or global (unqualified) name OverflowError On excessively large arithmetic operation RuntimeError Obsolete catch-all; define a suitable error instead SyntaxError On parser encountering a syntax error SystemError On non-fatal interpreter error - bug - report it SystemExit On `sys.exit()' TypeError On passing inappropriate type to built-in op or func ValueError On arg error not covered by TypeError or more precise ZeroDivisionError On division or modulo operation with 0 as 2nd arg **** Numbers **** @ Integers: 'C' long, >= 32 bits precision; OverflowError if bounds exceeded @ Long Integers: unlimited precision - '2147483648L' @ Floating point: machine double-precision floating point - '2147483648.0' Numeric operations vs special methods (s = self, o = other) s+o = __add__(s,o) s-o = __sub__(s,o) s*o = __mul__(s,o) s/o = __div__(s,o) s%o = __mod__(s,o) divmod(s,o) = __divmod__(s,o) pow(s,o) = __pow__(s,o) s&o = __and__(s,o) s^o = __xor__(s,o) s|o = __xor__(s,o) s<<o = __lshift__(s,o) s>>o = __rshift__(s,o) nonzero(s) = __nonzero__(s) coerce(s,o) = __coerce__(s,o) -s = __neg__(s) +s = __pos__(s) abs(s) = __abs__(s) ~s = __invert__(s) (bitwise) int(s) = __int__(s) long(s) = __long__(s) float(s) = __float__(s) oct(s) = __oct__(s) hex(s) = __hex__(s) Numeric functions: range(start=0,end,step=1) create arithmetic progression list round(x, n=0) round floating point x to n decimal places xrange(start=0,end,step=1) create virtual arithmetic progressions tuple Numeric exceptions: ~ TypeError: raised on application of arithemetic opertion to non-number ~ OverflowError: numeric bounds exceeded ~ ZeroDivisionError: raised when zero second argument of div or modulo op **** Collections - Sequences and Mappings **** Collections general operations vs methods (s: self, i: index or key) len(s) = __len__(s) length of object, >= 0. Length 0 == false s[i] = __getitem__(s,i) Element at index/key i, origin 0 Sequences @ String (immutable sequence): 'string contents' 'this is a string' "and so is this" ''' and this begins a 'multi-line' string with embedded quotes... and this is the end of that string.''' `expr` = __repr__(expr), converts arbitrary expr to string chr(int) string of ascii letter at ordinate in (0 <= int < 256) ord(string) ascii ordinate of string (string must be length 1) string % arg format operator, a la C sprintf arg for single directive can be any (suitable) type arg for multiple directives must be tuple or dict dict (mapping) arg uses parenthesized directives that are keys into it supports %, c, s, i, d, u, o, x, X, e, E, f, g, G directives * can be width and precision; directs use of corresponding (int) args * can not be used with dict args flag characters -, +, blank, #, and 0 understood. %s conversion takes any python object, converts using `str()' ANSI directives %p and %n not supported %s conversions do *not* take \000 as end of string @ Tuples (immutable sequence): (oneelem, another, etc) parens may be left off all but empty tuples singletons represented by affixing a comma to an expr empty tuple represented using empty parens @ Lists (mutable sequence): [oneelem, another, etc] assignment - must be 1-1 map of items in target and object sequences deletion - similar rules as for assignment List special methods: see mutable sequence ops vs methods, below Sequences general ops vs methods (s: self, i,j: indices, v: val) All collections general methods, plus: s[i:j] __getslice__(s,i,j), all s[k] s.t. i <= k < j min(s) smallest item of s max(s) largest item of s v [not] in s 1 if v [not] equal to an item in s, else 0 s + seq concatenation of s and seq s * num num copies of s concatenated, also, `num * s' Immutable sequence ops vs methods (s: self, i,j: indices, v: val) All collections and sequences general methods, plus: s[i:j] __getslice__(s,i,j), all s[k] s.t. i <= k < j ( For s[i:j], len(self) is intrinsically added to i, j < 0) ( Complex elems of immutable seqs may be mutable, see dictionaries, below) Mutable sequence ops vs methods (s: self, i,j: indices, v: val) All sequences' general methods, plus: ( for non-slice refs, i < 0 intrinsically has len(s) added) ( For slice refs, len(s) *is not* intrinsically added to i, j < 0) ( for assignment/deletion, index refs must point to existing items s[i]=v = __setitem__(s,i,v) del s[i] = __delitem__(s,i) s[i:j] = __getslice__(s,i,j) s[i:j]=seq = __setslice__(s,i,j,seq) del s[i:j] = __delslice__(s,i,j) == s[i:j] = [] s.append(seq) == `s[len(seq):len(seq)] = seq' (but faster) s.count(v) number of i's for which `s[i] == v' s.index(v) first i such that `s[i] == v', or IndexError if none s.insert(i, v) == `s[i:i] = [v]' s.remove(v) == `del s[s.index(v)]', or IndexError if v not in s s.reverse() reverse the items of s in place s.sort() permute s items so s[i] <= s[j], for i < j Mappings @ Dictionaries: {key1: val1, key2: val2, ...} built-in types as keys must be unalterable: obj & all contents immutable User-defined classes as keys must have __hash__() and __cmp__() methods ~ TypeError is raised if key not acceptable ~ KeyError is raised if reference made using non-existent key key types may vary (contrary to ref man) Dictionaries ops vs methods (s: self, k: key, v: val) all collections general ops, plus: hash(s) = __hash__(s) - hash value for dictionary references s[k]=v = __setitem__(s,k,v) del s[k] = __delitem__(s,k) s.items() = a list copy of s's (key, item) pairs s.keys() = a list copy of s's keys s.values() = a list copy of s's values s.has_keys(k) = 1 if s has key k, else 0 ( s.items, .keys, and .values yield random but mutually consistent order) **** Callables **** @ User defined functions: 'def name (param-list): suite' suite is not evaluated at statement execution, but at function invocation function parameters, comma separated on param-list: func_code: special attr, code object representing compiled function body func_globals: special attr, ref to global dict of funcs definition module func(arg-list) invocation @ User defined methods: like functions, with extra implicit arg Same as functions, but passed class instance as additional first argument im_self: special attr, method's class instance object im_func: special attr, function object mthd(args) invocation, same as mthd.im_func(mthd.im_self, args) @ Classes: 'class name[ (inheritance)]: suite' inheritance list is evaluated, if any, to identify base classes for name suite is executed in new local name space, which goes to the class object class name is bound in encompassing local name space container for dictionary containing class's ns dictionary __dict__: ro attr, class ns as dictionary object __bases__: ro attr, class' base classes in tuple __init__(self, args..): implements object instantiation __del__(self): implements impending object deletion @ Class instances __dict__: ro attr, class' attribute dictionary __class__: ro attr, instance's class object Callables special method vs ops (f: function) apply(f, args-tuple) call f with args-tuple as arg list compile(str, flnm, kind) compile string into exectuable code object eval(str, glbls=, lcls=) evaluate string as expression (cond_list) filter(f, seq) => seq of seq elems for which f is true map(f, lst, [lst2, ...]) => list of f applied to succesive lsts elems reduce(f, lst, initlzr) => value of f applied to elems and cume result @ **** Null object: `None' **** @ **** Type objects, print as: <type 'int'> **** ( '<...>' form is only for printing - cannot be entered that way, accessed via built-in func 'type()' ( equal only when identical (same id()), so can't just use the string name, must use object with the same str val) @ **** Modules **** functions and methods in a module share module's ("global") namespace function uses "global" statement to instantiate var in global context Modules use "import" to incorp other module's names - see Name Spaces Special attrs, methods, and operations __dict__: attr, module's ns as dictionary; can modify vals but not sruct __name__: ro attr, module's name as string import Instantiate module or module components within another reload(amod) Reread an imported module @ **** Files **** wrapper around a C stdio file pointer sys.stdin, sys.stdout, sys.stderr are standard io-stream file objects File operations: open(nm, mode='r', bufsize=sysdef) return new file object close() A closed file cannot be read or written anymore. flush() Flush the internal buffer, like stdio's `fflush()'. isatty() 1 if file connected to a tty(-like) device, else 0. read(SIZE) Read up to SIZE bytes frm file, less on EOF or no data readline() Read one entire line from the file. readlines() `readline()' til EOF and return list of lines read. seek(OFFSET,WHENCE) Set file's current position, like stdio's `fseek()' tell() Return file's current position, like stdio's `ftell()'. write(STR) Write a string to the file. There is no return value. File functions: input(prompt='') like raw input but accept '\' line continuations print exp, exp2, ... Write values to stdout raw_input(prompt='') prompt then read single line of input **** Internal types **** @ Code objects - represent exectuable code - function obj sans context @ Frame objects - represent executable frames - may occur in traceback objs @ Traceback objects - stack trace of an exception Control statements ================== * Calls and Evaluation * ( See also Callables, in Objects section above, for ops and methods.) exec: exec expr [ in expr2 [, expr3]] # Dynamic execution of python code return: return [expr] # Leave current func call with expr value * Conditionals and Loops * ( See also Boolean values, above) break: break # Terminate nearest enclosing loop continue: continue # Continue next cycle of enclosing loop if: if cond: suite [\n elif cond: suite \n ...] [\n else: suite] for: for targs in conds: suite [ \n else: suite ] while: while cond: suite [ \n else : suite ] * Exceptions * raise: raise expr [, expr2] # Raise exception expr, passing expr2 if any try... # Handle exceptions: try: suite [\n except [cond [, targ]]: suite \n ... \n] [else: suite] try: suite \n finally: suite # Specify a 'cleanup' handler ( two 'try' forms cannot be mixed together) System modules ============== @ * Built-ins * sys Interpreter state (see Operating Environment, above) __builtin__ Access to all built-in python identifiers __main__ Scope of the interpreters main program, script or stdin array Obj efficiently representing arrays of basic values math Math functions of C standard time Time-related functions regex Regular expression matching operations marshal Read and write some python values in binary format strop Miscellaneous string operations struct Convert between python values and C structs @ * Standard * getopt Parse cmd line args in sys.argv. A la UNIX 'getopt'. os Portable interface to OS dependent functionality pdb text-oriented debugger rand Pseudo-random generator, like C rand() regsub Functions useful for working with regular expressions string Useful string and characters functions and exceptions wdb window-oriented debugger whrandom Wichmann-Hill pseudo-random number generator @ * Miscellaneous * dis Python disassembler glob Filename globbing (a la unix shell) grep File string search posixpath Common operations on POSIX pathnames profile Python code profiler repr `...` repr operator with presentation constraints string All of builtin 'strop', plus tb Traceback browser and printer tempfile Produce a temporary-file name util Miscellaneous functions that don't belong elsewhere @ * Unix * dbm Dict/file-like interface to Unix ndbm database library grp Interface to Unix group database posix Standardized UNIX OS functionality - see 'os', above posixpath POSIX pathname functions - see 'os', above pwd Access to the Unix password database select Access to Unix select multiplex file synchronization socket Access to BSD socket interface thread Low-level primitives for working with process threads @ * Multimedia * audioop Useful operations on sound fragments imageop Useful operations on images jpeg Access to jpeg image compressor and decompressor rgbimg Access SGI imglib image files @ * Cryptographic Extensions * md5 Interface to RSA's MD5 message digest algorithm mpz Interface to int part of GNU multiple precision library rotor Implementation of a rotor-based encryption algorithm @ * Stdwin * Standard Window System stdwin Standard Window System interface stdwinevents Stdwin event, command, and selection constants rect Rectangle manipulation operations @ * SGI IRIX * (4 & 5) al SGI audio facilities AL al constants fl Interface to FORMS library FL fl constants flp Functions for form designer fm Access to font manager library gl Access to graphics library GL Constants for gl DEVICE More constants for gl imgfile Imglib image file interface @ * Suns * sunaudiodev Access to sun audio interface @ * Contrib * syslog metalbase Idioms and hints ================ Invoke main if running as script: if __name__ == '__main__': main() General object-type miscellany What type is someobj; eg, a list: if type(someobj) == type([]): Convert number to hex or octal string: hex(122) => '0x7a' Convert string to number: eval('0x7a') => 122 Sequence Slice conceptualization: +---+---+---+---+---+ Indices point ~between~ chars: | s | t | u | f | f | - first char's left edge = 0, +---+---+---+---+---+ - nth char's right edge = n. 0 1 2 3 4 5 (Note that -0 == 0, so it does -5 -4 -3 -2 -1 not count from right.) Sequences miscellany: Create merge of lists: map(None, lst1, lst2, ...) It's faster to list.append(elem) than to list[len(list):] = [elem] Copy structure of seq a to b: b = a[:] Comma (tuple vs expression grouping) nuances: to specify a tuple of one element: ('element',) to specify a tuple containing 1 tuple: (('contained', 'tuple'),) "TypeError: call of non-function" often means a list missing a comma Namespace and object surveillance: get dictful object keys/key-vals: dir(), vars() Current context globals: eval(__name__ + '.__dict__') Current context locals: vars() Methods supported by X (some objs): X.__methods__ X's data attributes (some objs): X.__members__