Build in functions Flashcards
compile(source, filename,
mode[, flags[, dont_inherit]])
Compile the source into a code or AST object. Code objects can be executed by an exec statement or evaluated by a call to eval(). source can either be a Unicode string, a Latin-1 encoded string or an AST object. Refer to the ast module documentation for information on how to work with AST objects.
The filename argument should give the file from which the code was read; pass some recognizable value if it wasn’t read from a file (’’ is commonly used).
The mode argument specifies what kind of code must be compiled; it can be ‘exec’ if source 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 other than None will be printed).
The optional arguments flags and dont_inherit control which future statements (see PEP 236) affect the compilation of source. If neither is present (or both are zero) the code is compiled with those future statements that are in effect in the code that is calling compile. If the flags argument is given and dont_inherit is not (or is zero) then the future statements specified by the flags argument are used in addition to those that would be used anyway. If dont_inherit is a non-zero integer then the flags argument is it – the future statements in effect around the call to compile are ignored.
Future statements are specified by bits which can be bitwise ORed together to specify multiple statements. The bitfield required to specify a given feature can be found as the compiler_flag attribute on the _Feature instance in the __future__ module.
This function raises SyntaxError if the compiled source is invalid, and TypeError if the source contains null bytes.
Note:
When compiling a string with multi-line code in ‘single’ or ‘eval’ mode, input must be terminated by at least one newline character. This is to facilitate detection of incomplete and complete statements in the *code *module.
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( ), otherwise the representation is a string enclosed in angle brackets that contains the name of the type of the object together with additional information often including the name and address of the object. A class can control what this function returns for its instances by defining a __repr__( ) method.
bin(x)
Convert an integer number to a binary string. The result is a valid Python expression. If x is not a Python int** **object, it has to define an __index__() method that returns an integer.
hash(object)
Return the hash value of the object (if it has one). Hash values are 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, as is the case for 1 and 1.0).
input([prompt])
Equivalent to *eval *( raw_input(prompt) ).
This function does not catch user errors. If the input is not syntactically valid, a *SyntaxError *will be raised. Other exceptions may be raised if there is an error during evaluation.
If the *readline *module was loaded, then input() will use it to provide elaborate line editing and history features.
Consider using the raw_input() function for general input from users.
list([iterable])
Return a list whose items are the same and in the same order as iterable‘s items. iterable may be either a sequence, a container that supports iteration, or an iterator object. If iterable is already a list, a copy is made and returned, similar to *iterable[:] *. For instance, list(‘abc’) returns [‘a’, ‘b’, ‘c’] and list( (1, 2, 3)) returns [1, 2, 3]. If no argument is given, returns a new empty list, *[] *.
*list *is a mutable sequence type, as documented in Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange. For other containers see the built in dict, set, and *tuple *classes, and the *collections *module.
locals()
Update and return a dictionary representing the current local symbol table. Free variables are returned by locals() when it is called in function blocks, but not in class blocks.
range(stop)
range(start, stop[, 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 stepargument 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 stop; if step is negative, the last element is the smallest start + i * step greater than stop. step must not be zero (or else ValueError is raised). Example:
hasattr(object, name)
The arguments are an object and a string. The result is True if the string is the name of one of the object’s attributes, *False *if not. (This is implemented by calling getattr(object, name) and seeing whether it raises an exception or not.)
int(x=0)
int(x, base=10)
Convert a number or string x to an integer, or return 0 if no arguments are given. If x is a number, it can be a plain integer, a long integer, or a floating point number. If x is floating point, the conversion truncates towards zero. If the argument is outside the integer range, the function returns a long object instead.
If x is not a number or if base is given, then x must be a string or Unicode object representing an integer literal in radix base. Optionally, the literal can be preceded by + or - (with no space in between) and surrounded by whitespace. A base-n literal consists of the digits 0 to n-1, with a to z (or A to Z) having values 10 to 35. The default base is 10. The allowed values are 0 and 2-36. Base-2, -8, and -16 literals can be optionally prefixed with 0b/0B, 0o/0O/0, or 0x/0X, as with integer literals in code. Base 0 means to interpret the string exactly as an integer literal, so that the actual base is 2, 8, 10, or 16.
The integer type is described in Numeric Types — int, float, long, complex.
zip([iterable, …])
This function returns a list of tuples, where the i-th tuple contains the i-th element from each of the argument sequences or iterables. The returned list is truncated in length to the length of the shortest argument sequence. When there are multiple arguments which are all of the same length, zip( ) is similar to map( ) with an initial argument of None. With a single sequence argument, it returns a list of 1-tuples. With no arguments, it returns an empty list.
The left-to-right evaluation order of the iterables is guaranteed. This makes possible an idiom for clustering a data series into n-length groups using zip(*[iter(s)]*n) .
zip( ) in conjunction with the * operator can be used to unzip a list:
(See Bottom Picture)
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).
dict(**kwarg)
dict(mapping, **kwarg)
dict(iterable, **kwarg)
Create a new dictionary. The dict object is the dictionary class. See dict and Mapping Types — dict for documentation about this class.
For other containers see the built-in list, set, and tuple classes, as well as the collections module.
chr(i)
Return a string of one character whose ASCII code is the integer i. For example, chr(97) returns the string ‘a’. This is the inverse of ord(). The argument must be in the range [0..255], inclusive; ValueError will be raised if i is outside that range. See also unichr().
memoryview(obj)
Return a “memory view” object created from the given argument. See memoryview type for more information.
round(number[, ndigits])
Return the floating point value number rounded to ndigits digits after the decimal point. If ndigits 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 ndigits; if two multiples are equally close, rounding is done away from 0 (so. for example, round(0.5) is 1.0 and round(-0.5) is *-1.0 *).
Note
The behavior of round( ) for floats can be surprising: for example, round(2.675, 2) gives 2.67 instead of the expected 2.68. This is not a bug: it’s a result of the fact that most decimal fractions can’t be represented exactly as a float. See Floating Point Arithmetic: Issues and Limitations for more information.
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 *, zero if x == y and strictly positive if x > y.
Self note: if x < y, returns -1, if x == y, returns 0, if x > y, returns 1.
eval(expression[, globals[, locals]])
The arguments are a Unicode or Latin-1 encoded string and optional globals and locals. If provided, globals must be a dictionary. If provided, locals can be any mapping object.
Changed in version 2.4: formerly locals was required to be a dictionary.
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 namespace. If the globals dictionary is present and lacks ‘__builtins__’, the current globals are copied into globals before expression is parsed. This means that expression normally has full access to the standard __builtin__ module and restricted environments are propagated. 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 (See Bottom Picture):
This function can also be used to execute arbitrary code objects (such as those created by compile() ). In this case pass a code object instead of a string. If the code object has been compiled with ‘exec’ as the mode argument, eval() ‘s return value will be None.
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().
See ast.literal_eval() for a function that can safely evaluate strings with expressions containing only literals.
bool([x])
Convert a value to a Boolean, using the standard truth testing procedure. If x is false or omitted, this returns False; otherwise it returns True. *bool *is also a class, which is a subclass of int. Class bool cannot be subclassed further. Its only instances are False and True.
format(value[, format_spec])
Convert a value to a “formatted” representation, as controlled by format_spec. The interpretation of format_spec will depend on the type of the value argument, however there is a standard formatting syntax that is used by most built-in types: Format Specification Mini-Language.
Note
format(value, format_spec) merely calls value.__format__(format_spec).
next(iterator[, default])
Retrieve the next item from the iterator by calling its next() method. If default is given, it is returned if the iterator is exhausted, otherwise StopIteration is raised.
enumerate(sequence, start=0)
Return an enumerate object. sequence must be a sequence, an iterator, or some other object which supports iteration. The next() method of the iterator returned by enumerate() returns a tuple containing a count (from start which defaults to 0) and the values obtained from iterating over sequence:
basestring()
This abstract type is the superclass for str and unicode. It cannot be called or instantiated, but it can be used to test whether an object is an instance of str or unicode. isinstance(obj, basestring) is equivalent to isinstance(obj, (str, unicode)).
complex([real[, imag]])
Create a complex number with the value real + imag*j or convert a string or number to a complex number. If the first parameter is a string, it will be interpreted as a complex number and the function must be called without a second parameter. The second parameter can never be a string. Each argument may be any numeric type (including complex). If imag is omitted, it defaults to zero and the function serves as a numeric conversion function like int(), long() and float(). If both arguments are omitted, returns 0j.
Note:
When converting from a string, the string must not contain whitespace around the central + or - operator. For example, complex(‘1+2j’) is fine, butcomplex(‘1 + 2j’) raises ValueError.
The complex type is described in Numeric Types — int, float, long, complex.
unicode(object=’’)
unicode(object[, encoding[, errors]])
Return the Unicode string version of object using one of the following modes:
If encoding and/or errors are given, unicode( ) will decode the object which can either be an 8-bit string or a character buffer using the codec for encoding. The encoding parameter is a string giving the name of an encoding; if the encoding is not known, LookupError is raised. Error handling is done according to errors; this specifies the treatment of characters which are invalid in the input encoding. If errors is ‘strict’ (the default), a ValueError is raised on errors, while a value of ‘ignore’ causes errors to be silently ignored, and a value of ‘replace’ causes the official Unicode replacement character, U+FFFD, to be used to replace input characters which cannot be decoded. See also the codecs module.
If no optional parameters are given, unicode( ) will mimic the behaviour of str( ) except that it returns Unicode strings instead of 8-bit strings. More precisely, if object is a Unicode string or subclass it will return that Unicode string without any additional decoding applied.
For objects which provide a __unicode__( ) method, it will call this method without arguments to create a Unicode string. For all other objects, the 8-bit string version or representation is requested and then converted to a Unicode string using the codec for the default encoding in ‘strict’ mode.
For more information on Unicode strings see Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange which describes sequence functionality (Unicode strings are sequences), and also the string-specific methods described in the String Methods section. To output formatted strings use template strings or the % operator described in the String Formatting Operations section. In addition see the String Services section. See also str( ).
max(iterable[, key])
max(arg1, arg2, *args[, key])
Return the largest item in an iterable or the largest of two or more arguments.
If one positional argument is provided, iterable must be a non-empty iterable (such as a non-empty string, tuple or list). The largest item in the iterable is returned. If two or more positional arguments are provided, the largest of the positional arguments is returned.
The optional key argument specifies a one-argument ordering function like that used for list.sort(). The key argument, if supplied, must be in keyword form (for example, *max(a,b,c,key=func) *).
unichr(i)
Return the Unicode string of one character whose Unicode code is the integer i. For example, unichr(97) returns the string u’a’. This is the inverse of ord( ) for Unicode strings. The valid range for the argument depends how Python was configured – it may be either UCS2 [0..0xFFFF] or UCS4 [0..0x10FFFF]. *ValueError *is raised otherwise. For ASCII and 8-bit strings see chr( ).
reduce(function, iterable[, initializer])
Apply function of two arguments cumulatively to the items of iterable, from left to right, so as to reduce the iterable to a single value.
For example, reduce(lambda x,y: x+y, [1, 2, 3, 4, 5]) calculates ((((1+2)+3)+4)+5). The left argument, x, is the accumulated value and the right argument, y, is the update value from the iterable. If the optional initializer is present, it is placed before the items of the iterable in the calculation, and serves as a default when the iterable is empty. If initializer is not given and iterable contains only one item, the first item is returned. Roughly equivalent to:
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. If no argument is given, returns the empty string, ’ ‘.
For more information on strings see Sequence Types — str, unicode, list, tuple, bytearray, buffer, xrange which describes sequence functionality (strings are sequences), and also the string-specific methods described in the String Methods section. To output formatted strings use template strings or the % operator described in the String Formatting Operations section. In addition see the String Services section. See also unicode( )
min(iterable[, key])
min(arg1, arg2, *args[, key])
Return the smallest item in an iterable or the smallest of two or more arguments.
If one positional argument is provided, iterable must be a non-empty iterable (such as a non-empty string, tuple or list). The smallest item in the iterable is returned. If two or more positional arguments are provided, the smallest of the positional arguments is returned.
The optional key argument specifies a one-argument ordering function like that used for list.sort(). The key argument, if supplied, must be in keyword form (for example, *min(a,b,c,key=func) *).
all(iterable)
Return True if all elements of the iterable are true (or if the iterable is empty). Equivalent to:
iter(o[, sentinel])
Return an iterator object. The first argument is interpreted very differently depending on the presence of the second argument. Without a second argument, o must be a collection object which supports the iteration protocol (the __iter__() method), or it must support the sequence protocol (the __getitem__() method with integer arguments starting at 0). If it does not support either of those protocols, TypeError is raised. If the second argument, sentinel, is given, then o must be a callable object. The iterator created in this case will call o with no arguments for each call to its next() method; if the value returned is equal to sentinel, StopIteration will be raised, otherwise the value will be returned.
One useful application of the second form of iter() is to read lines of a file until a certain line is reached. The following example reads a file until the readline() method returns an empty string:
dir([object])
Without arguments, return the list of names in the current local scope. With an argument, attempt to return a list of valid attributes for that object.
If the object has a method named __dir__(), this method will be called and must return the list of attributes. This allows objects that implement a custom __getattr__() or __getattribute__() function to customize the way dir() reports their attributes.
If the object does not provide __dir__(), the function tries its best to gather information from the object’s __dict__ attribute, if defined, and from its type object. The resulting list is not necessarily complete, and may be inaccurate when the object has a custom __getattr__().
The default dir() mechanism behaves differently with different types of objects, as it attempts to produce the most relevant, rather than complete, information:
- If the object is a module object, the list contains the names of the module’s attributes.
- If the object is a type or class object, the list contains the names of its attributes, and recursively of the attributes of its bases.
- Otherwise, the list contains the object’s attributes’ names, the names of its class’s attributes, and recursively of the attributes of its class’s base classes.
Note:
Because dir() is supplied primarily as a convenience for use at an interactive prompt, it tries to supply an interesting set of names more than it tries to supply a rigorously or consistently defined set of names, and its detailed behavior may change across releases. For example, metaclass attributes are not in the result list when the argument is a class.
The resulting list is sorted alphabetically. For example(See Bottom Picture):
property([fget[, fset[, fdel[, doc]]]])
Return a property attribute for new-style classes (classes that derive from object ).
fget is a function for getting an attribute value, likewise fset is a function for setting, and fdel a function for del’ing, an attribute. Typical use is to define a managed attribute x
(See Class C First):
If then c is an instance of C, c.x will invoke the getter, c.x = value will invoke the setter and del c.x the deleter.
If given, doc will be the docstring of the property attribute. Otherwise, the property will copy fget‘s docstring (if it exists). This makes it possible to create read-only properties easily using property( ) as a decorator:
(See Class Parrot)
turns the voltage( ) method into a “getter” for a read-only attribute with the same name.
A property object has getter, setter, and deleter methods usable as decorators that create a copy of the property with the corresponding accessor function set to the decorated function. This is best explained with an example:
(See Class C Last)
This code is exactly equivalent to the first example. Be sure to give the additional functions the same name as the original property (x in this case.)
The returned property also has the attributes fget, fset, and fdel corresponding to the constructor arguments.
classmethod(function)
Return a class method for function.
A class method receives the class as implicit first argument, just like an instance method receives the instance. To declare a class method, use this idiom(See Bottom Picture):
The @classmethod form is a function decorator – see the description of function definitions in Function definitions for details.
It can be called either on the class (such as C.f() ) or on an instance (such as C().f() ). The instance is ignored except for its class. If a class method is called for a derived class, the derived class object is passed as the implied first argument.
Class methods are different than C++ or Java static methods. If you want those, see staticmethod() in this section.
For more information on class methods, consult the documentation on the standard type hierarchy in The standard type hierarchy.
super(type[, object-or-type])
Return a proxy object that delegates method calls to a parent or sibling class of type. This is useful for accessing inherited methods that have been overridden in a class. The search order is same as that used by getattr() except that the type itself is skipped.
The __mro__ attribute of the type lists the method resolution search order used by both getattr() and super(). The attribute is dynamic and can change whenever the inheritance hierarchy is updated.
If the second argument is omitted, the super object returned is unbound. If the second argument is an object, isinstance(obj, type) must be true. If the second argument is a type, issubclass(type2, type) must be true (this is useful for classmethods).
Note super( ) only works for new-style classes.
There are two typical use cases for super. In a class hierarchy with single inheritance, super can be used to refer to parent classes without naming them explicitly, thus making the code more maintainable. This use closely parallels the use of super in other programming languages.
The second use case is to support cooperative multiple inheritance in a dynamic execution environment. This use case is unique to Python and is not found in statically compiled languages or languages that only support single inheritance. This makes it possible to implement “diamond diagrams” where multiple base classes implement the same method. Good design dictates that this method have the same calling signature in every case (because the order of calls is determined at runtime, because that order adapts to changes in the class hierarchy, and because that order can include sibling classes that are unknown prior to runtime).
For both use cases, a typical superclass call looks like this:
(See Bottom Picture)
Note that super( ) is implemented as part of the binding process for explicit dotted attribute lookups such as super( ).__getitem__(name). It does so by implementing its own __getattribute__( ) method for searching classes in a predictable order that supports cooperative multiple inheritance. Accordingly, super( ) is undefined for implicit lookups using statements or operators such as super( )[name] .
Also note that super( ) is not limited to use inside methods. The two argument form specifies the arguments exactly and makes the appropriate references.
For practical suggestions on how to design cooperative classes using super( ), see guide to using super( ).
print(*objects, sep=’’,
end=’\n’, file=sys.stdout)
Print objects to the stream file, separated by sep and followed by end. sep, end and file, if present, must be given as keyword arguments.
All non-keyword arguments are converted to strings like str() does and written to the stream, separated by sep and followed by end. Both sep and end must be strings; they can also be None, which means to use the default values. If no objects are given, print( ) will just write end.
The file argument must be an object with a write(string) method; if it is not present or None, sys.stdout will be used. Output buffering is determined by file. Use file.flush( ) to ensure, for instance, immediate appearance on a screen.
Note
This function is not normally available as a built-in since the name print is recognized as the print statement. To disable the statement and use the
print( ) function, use this future statement at the top of your module:
reload(module)
Reload a previously 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 (the same as the module argument).
When reload(module) is executed:
- Python modules’ code is recompiled and the module-level code reexecuted, defining a new set of objects which are bound to names in the module’s dictionary. The init function of extension modules is not called a second time.
- As with all other objects in Python the old objects are only reclaimed after their reference counts drop to zero.
* The names in the module namespace are updated to point to any new or changed objects. dsa
- Other references to the old objects (such as names external to the module) are not rebound to refer to the new objects and must be updated in each namespace where they occur if that is desired.
There are a number of other 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:
(See Bottom Picture)
It is legal though generally not very useful to reload built-in or dynamically loaded modules, except for sys, __main__ and __builtin__. In many 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.
type(object)
type(name, bases, dict)
With one argument, return the type of an object. The return value is a type object. The isinstance( ) built-in function is recommended for testing the type of an object.
With three arguments, return a new type object. This is essentially a dynamic form of the class statement. The name string is the class name and becomes the __name__ attribute; the bases tuple itemizes the base classes and becomes the __bases__ attribute; and the dict dictionary is the namespace containing definitions for class body and becomes the __dict__ attribute. For example, the following two statements create identical type objects:
any(iterable)
Return True if any element of the iterable is true. If the iterable is empty, return False. Equivalent to:
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:
(See Bottom Image)
If the readline module was loaded, then raw_input( ) will use it to provide elaborate line editing and history features.
