That should be all for now!!
Ken
ken.manheimer@nist.gov, 301 975-3539)
A Python Bestiary
Itemizing Python Objects and Nuances
$Revision: 1.5 $ $Date: 1994/07/26 20:16:28 $ 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__