__getitem__

object.__getitem__(self,key)

class MyClass:
    def __getitem__(self, key):
	self.key = key
        print(self.key)
        return None

__getitem__(self, key)

Defines behaviour for when an item is accessed, using the notation self[key]. This is also part of both the mutable and immutable container protocols.

It should also raise appropriate exceptions:

TypeError if the type of the key is wrong

KeyError if there is no corresponding value for the key.

Called to implement evaluation of self[key].

For sequence types, the accepted keys should be integers and slice objects. Note that the special interpretation of negative indexes (if the class wishes to emulate a sequence type) is up to the __getitem__() method.

If key is of an inappropriate type, TypeError may be raised;

if of a value outside the set of indexes for the sequence (after any special interpretation of negative values), IndexError should be raised.

For mapping types, if key is missing (not in the container), KeyError should be raised.

Note for loops expect that an IndexError will be raised for illegal indexes to allow proper detection of the end of the sequence.

object.__reversed__(self)

Called (if present) by the reversed() built-in to implement reverse iteration.

It should return a new iterator object that iterates over all the objects in the container in reverse order.

If the __reversed__() method is not provided, the reversed() built-in will fall back to using the sequence protocol (__len__() and __getitem__()).

Objects that support the sequence protocol should only provide __reversed__() if they can provide an implementation that is more efficient than the one provided by reversed().

Slice of py

classslice(stop)classslice(start,stop[,step])

Return a slice object representing the set of indices specified by range(start, stop, step).

The start and step arguments default to None. Slice objects have read-only data attributes start, stop and step which merely return the argument values (or their default).

They have no other explicit functionality; however they are used by Numerical Python and other third party extensions.

Slice objects are also generated when extended indexing syntax is used.

For example:

a[start:stop:step] or a[start:stop, i].

start

Optional. An integer number specifying at which position to start the slicing. Default is 0

stop

An integer number specifying at which position to end the slicing (Not inclusive)

step

Optional. An integer number specifying the step of the slicing. Default is 1

Negative Indexing

When slicing in reverse it is better to use the negative indice values of the items in the element being sliced.

Where positive values are being used for a reversed slice, with indice[0] required to be part of the slice.. Then do not use -1, as the stop value. Instead leave the value None.

-1 would be assumed by python to be part of a negative indice[-1] the item of the element last on the right. This would then give an empty slice.

(3:-1:-1) would wrongly be seen as start at 3, stop at -1 with -1 step(going right to left)

This would start indice [3], try to go right to left to go to -1. As indice[-1] is to the right of 3 an error would be raised.

Slicings

A slicing selects a range of items in a sequence object (e.g., a string, tuple or list).

Slicings may be used as expressions or as targets in assignment or del statements.

The syntax for a slicing:
slicing      ::=  primary "[" slice_list "]"
slice_list   ::=  slice_item ("," slice_item)* [","]
slice_item   ::=  expression | proper_slice
proper_slice ::=  [lower_bound] ":" [upper_bound] [ ":" [stride] ]
lower_bound  ::=  expression
upper_bound  ::=  expression
stride       ::=  expression

There is ambiguity in the formal syntax here: anything that looks like an expression list also looks like a slice list, so any subscription can be interpreted as a slicing.

Rather than further complicating the syntax, this is disambiguated by defining that in this case the interpretation as a subscription takes priority over the interpretation as a slicing (this is the case if the slice list contains no proper slice).

The semantics for a slicing are as follows.

The primary is indexed (using the same __getitem__() method as normal subscription) with a key that is constructed from the slice list, as follows.

If the slice list contains at least one comma, the key is a tuple containing the conversion of the slice items; otherwise, the conversion of the lone slice item is the key.

The conversion of a slice item that is an expression is that expression.

The conversion of a proper slice is a slice object whose start, stop and step attributes are the values of the expressions given as lower bound, upper bound and stride, respectively, substituting None for missing expressions.

Mutable Sequence Types

The operations in the following table are defined on mutable sequence types.

The collections.abc.MutableSequence ABC is provided to make it easier to correctly implement these operations on custom sequence types.

In the table s is an instance of a mutable sequence type, t is any iterable object and x is an arbitrary object that meets any type and value restrictions imposed by s

(for example, bytearray only accepts integers that meet the value restriction 0 <= x <= 255).

Operation Result Notes

s[i] = x item i of s is replaced by x

s[i:j] = t slice of s from i to j is replaced by the contents of the iterable t

del s[i:j] same as s[i:j] = []

s[i:j:k] = t the elements of s[i:j:k] are replaced by those of t (1)

del s[i:j:k] removes the elements of s[i:j:k] from the list

s.append(x) appends x to the end of the sequence (same as s[len(s):len(s)] = [x])

s.clear() removes all items from s (same as del s[:]) (5)

s.copy() creates a shallow copy of s (same as s[:]) (5)

s.extend(t) or s += t extends s with the contents of t (for the most part the same as s[len(s):len(s)] = t)

s *= n updates s with its contents repeated n times (6)

s.insert(i, x) inserts x into s at the index given by i (same as s[i:i] = [x])

s.pop([i]) retrieves the item at i and also removes it from s (2)

s.remove(x) remove the first item from s where s[i] is equal to x (3)

s.reverse() reverses the items of s in place (4)

Notes:

1. t must have the same length as the slice it is replacing.

2. The optional argument i defaults to -1, so that by default the last item is removed and returned.

3. remove() raises ValueError when x is not found in s.

4. The reverse() method modifies the sequence in place for economy of space when reversing a large sequence. To remind users that it operates by side effect, it does not return the reversed sequence.

5. clear() and copy() are included for consistency with the interfaces of mutable containers that don’t support slicing operations (such as dict and set). copy() is not part of the collections.abc.MutableSequence ABC, but most concrete mutable sequence classes provide it. New in version 3.3: clear() and copy() methods.

6. The value n is an integer, or an object implementing __index__(). Zero and negative values of n clear the sequence. Items in the sequence are not copied; they are referenced multiple times, as explained for s * n under Common Sequence Operations.

PEP 3132- Extended Iterable Unpacking

The specification for the *target feature.

struct - Interpret strings as packed binary data

Packing and Unpacking

This module performs conversions between Python values and C structs represented as Python strings. This can be used in handling binary data stored in files or from network connections, among other sources. It uses Format Strings as compact descriptions of the layout of the C structs and the intended conversion to/from Python values.

Note By default, the result of packing a given C struct includes pad bytes in order to maintain proper alignment for the C types involved; similarly, alignment is taken into account when unpacking. This behaviour is chosen so that the bytes of a packed struct correspond exactly to the layout in memory of the corresponding C struct. To handle platform-independent data formats or omit implicit pad bytes, use standard size and alignment instead of native size and alignment: see Byte Order, Size, and Alignment for details.

Functions and Exceptions The module defines the following exception and functions: exception struct.error

Exception raised on various occasions; argument is a string describing what is wrong.

struct.pack(fmt, v1, v2, ...)

Return a string containing the values v1, v2, ... packed according to the given format. The arguments must match the values required by the format exactly.

struct.pack_into(fmt, buffer, offset, v1, v2, ...)

Pack the values v1, v2, ... according to the given format, write the packed bytes into the writable buffer starting at offset.

Note that the offset is a required argument. New in version 2.5.

struct.unpack(fmt, string)

Unpack the string (presumably packed by pack(fmt, ...)) according to the given format. The result is a tuple even if it contains exactly one item. The string must contain exactly the amount of data required by the format (len(string) must equal calcsize(fmt)).

struct.unpack_from(fmt, buffer[, offset=0])

Unpack the buffer according to the given format. The result is a tuple even if it contains exactly one item. The buffer must contain at least the amount of data required by the format (len(buffer[offset:]) must be at least calcsize(fmt)). New in version 2.5.

struct.calcsize(fmt)

Return the size of the struct (and hence of the string) corresponding to the given format.

Format Strings Format strings are the mechanism used to specify the expected layout when packing and unpacking data. They are built up from Format Characters, which specify the type of data being packed/unpacked. In addition, there are special characters for controlling the Byte Order, Size, and Alignment.

Byte Order, Size, and Alignment By default, C types are represented in the machine’s native format and byte order, and properly aligned by skipping pad bytes if necessary (according to the rules used by the C compiler). Alternatively, the first character of the format string can be used to indicate the byte order, size and alignment of the packed data, according to the following table:

CharacterByte orderSizeAlignment@nativenativenative=nativestandardnone<little-endianstandardnone>big-endianstandardnone!network (= big-endian)standardnone

If the first character is not one of these, '@' is assumed. Native byte order is big-endian or little-endian, depending on the host system.

For example, Intel x86 and AMD64 (x86-64) are little-endian; Motorola 68000 and PowerPC G5 are big-endian; ARM and Intel Itanium feature switchable endianness (bi-endian).

Use sys.byteorder to check the endianness of your system.

Native size and alignment are determined using the C compiler’s sizeof expression. This is always combined with native byte order. Standard size depends only on the format character; see the table in the Format Characters section. Note the difference between '@' and '=': both use native byte order, but the size and alignment of the latter is standardised. The form '!' is available for those poor souls who claim they can’t remember whether network byte order is big-endian or little-endian. There is no way to indicate non-native byte order (force byte-swapping); use the appropriate choice of '<' or '>'.

Notes:

Padding is only automatically added between successive structure members. No padding is added at the beginning or the end of the encoded struct.

No padding is added when using non-native size and alignment, e.g. with ‘<’, ‘>’, ‘=’, and ‘!’.

To align the end of a structure to the alignment requirement of a particular type, end the format with the code for that type with a repeat count of zero.

Format Characters Format characters have the following meaning; the conversion between C and Python values should be obvious given their types. The ‘Standard size’ column refers to the size of the packed value in bytes when using standard size; that is, when the format string starts with one of '<', '>', '!' or '='. When using native size, the size of the packed value is platform-dependent.

Notes:

The '?' conversion code corresponds to the _Bool type defined by C99. If this type is not available, it is simulated using a char. In standard mode, it is always represented by one byte. New in version 2.6.

The 'q' and 'Q' conversion codes are available in native mode only if the platform C compiler supports C long long, or, on Windows, __int64. They are always available in standard modes. New in version 2.2.

When attempting to pack a non-integer using any of the integer conversion codes, if the non-integer has a __index__() method then that method is called to convert the argument to an integer before packing. If no __index__() method exists, or the call to __index__() raises TypeError, then the __int__() method is tried. However, the use of __int__() is deprecated, and will raise DeprecationWarning. Changed in version 2.7: Use of the __index__() method for non-integers is new in 2.7. Changed in version 2.7: Prior to version 2.7, not all integer conversion codes would use the __int__() method to convert, and DeprecationWarning was raised only for float arguments.

For the 'f' and 'd' conversion codes, the packed representation uses the IEEE 754 binary32 (for 'f') or binary64 (for 'd') format, regardless of the floating-point format used by the platform.

The 'P' format character is only available for the native byte ordering (selected as the default or with the '@' byte order character). The byte order character '=' chooses to use little- or big-endian ordering based on the host system. The struct module does not interpret this as native ordering, so the 'P' format is not available.

A format character may be preceded by an integral repeat count. For example, the format string '4h' means exactly the same as 'hhhh'. Whitespace characters between formats are ignored; a count and its format must not contain whitespace though.

For the 's' format character, the count is interpreted as the size of the string, not a repeat count like for the other format characters; for example, '10s' means a single 10-byte string, while '10c' means 10 characters. If a count is not given, it defaults to 1. For packing, the string is truncated or padded with null bytes as appropriate to make it fit. For unpacking, the resulting string always has exactly the specified number of bytes. As a special case, '0s' means a single, empty string (while '0c' means 0 characters).

The 'p' format character encodes a “Pascal string”, meaning a short variable-length string stored in a fixed number of bytes, given by the count. The first byte stored is the length of the string, or 255, whichever is smaller. The bytes of the string follow. If the string passed in to pack() is too long (longer than the count minus 1), only the leading count-1 bytes of the string are stored. If the string is shorter than count-1, it is padded with null bytes so that exactly count bytes in all are used. Note that for unpack(), the 'p' format character consumes count bytes, but that the string returned can never contain more than 255 characters.

For the 'P' format character, the return value is a Python integer or long integer, depending on the size needed to hold a pointer when it has been cast to an integer type. A NULL pointer will always be returned as the Python integer 0. When packing pointer-sized values, Python integer or long integer objects may be used. For example, the Alpha and Merced processors use 64-bit pointer values, meaning a Python long integer will be used to hold the pointer; other platforms use 32-bit pointers and will use a Python integer.

For the '?' format character, the return value is either True or False. When packing, the truth value of the argument object is used. Either 0 or 1 in the native or standard bool representation will be packed, and any non-zero value will be True when unpacking.

Examples Note All examples assume a native byte order, size, and alignment with a big-endian machine. A basic example of packing/unpacking three integers:

>>> from struct import * >>> pack('hhl', 1, 2, 3) '\x00\x01\x00\x02\x00\x00\x00\x03' >>> unpack('hhl', '\x00\x01\x00\x02\x00\x00\x00\x03') (1, 2, 3) >>> calcsize('hhl') 8

Unpacked fields can be named by assigning them to variables or by wrapping the result in a named tuple:

>>> record = 'raymond \x32\x12\x08\x01\x08' >>> name, serialnum, school, gradelevel = unpack('<10sHHb', record) >>> from collections import namedtuple >>> Student = namedtuple('Student', 'name serialnum school gradelevel') >>> Student._make(unpack('<10sHHb', record)) Student(name='raymond ', serialnum=4658, school=264, gradelevel=8)

The ordering of format characters may have an impact on size since the padding needed to satisfy alignment requirements is different:

>>> pack('ci', '*', 0x12131415) '*\x00\x00\x00\x12\x13\x14\x15' >>> pack('ic', 0x12131415, '*') '\x12\x13\x14\x15*' >>> calcsize('ci') 8 >>> calcsize('ic') 5

The following format 'llh0l' specifies two pad bytes at the end, assuming longs are aligned on 4-byte boundaries:

>>> pack('llh0l', 1, 2, 3) '\x00\x00\x00\x01\x00\x00\x00\x02\x00\x03\x00\x00'

This only works when native size and alignment are in effect; standard size and alignment does not enforce any alignment.

Classes The struct module also defines the following type: class struct.Struct(format)

Return a new Struct object which writes and reads binary data according to the format string format. Creating a Struct object once and calling its methods is more efficient than calling the struct functions with the same format since the format string only needs to be compiled once. New in version 2.5.

Compiled Struct objects support the following methods and attributes: pack(v1, v2, ...)

Identical to the pack() function, using the compiled format. (len(result) will equal self.size.)

pack_into(buffer, offset, v1, v2, ...)

Identical to the pack_into() function, using the compiled format.

unpack(string)Identical to the unpack() function, using the compiled format. (len(string) must equal self.size).

unpack_from(buffer, offset=0)

Identical to the unpack_from() function, using the compiled format. (len(buffer[offset:])

must be at least self.size).format

The format string used to construct this Struct object.size

The calculated size of the struct (and hence of the string) corresponding to format.