842 lines
32 KiB
TeX
842 lines
32 KiB
TeX
\section{Built-in Types}
|
|
|
|
The following sections describe the standard types that are built into
|
|
the interpreter. These are the numeric types, sequence types, and
|
|
several others, including types themselves. There is no explicit
|
|
Boolean type; use integers instead.
|
|
\indexii{built-in}{types}
|
|
\indexii{Boolean}{type}
|
|
|
|
Some operations are supported by several object types; in particular,
|
|
all objects can be compared, tested for truth value, and converted to
|
|
a string (with the \code{`{\rm \ldots}`} notation). The latter conversion is
|
|
implicitly used when an object is written by the \code{print} statement.
|
|
\stindex{print}
|
|
|
|
\subsection{Truth Value Testing}
|
|
|
|
Any object can be tested for truth value, for use in an \code{if} or
|
|
\code{while} condition or as operand of the Boolean operations below.
|
|
The following values are considered false:
|
|
\stindex{if}
|
|
\stindex{while}
|
|
\indexii{truth}{value}
|
|
\indexii{Boolean}{operations}
|
|
\index{false}
|
|
|
|
\begin{itemize}
|
|
\setindexsubitem{(Built-in object)}
|
|
|
|
\item \code{None}
|
|
\ttindex{None}
|
|
|
|
\item zero of any numeric type, e.g., \code{0}, \code{0L}, \code{0.0}.
|
|
|
|
\item any empty sequence, e.g., \code{''}, \code{()}, \code{[]}.
|
|
|
|
\item any empty mapping, e.g., \code{\{\}}.
|
|
|
|
\item instances of user-defined classes, if the class defines a
|
|
\code{__nonzero__()} or \code{__len__()} method, when that
|
|
method returns zero.
|
|
|
|
\end{itemize}
|
|
|
|
All other values are considered true --- so objects of many types are
|
|
always true.
|
|
\index{true}
|
|
|
|
Operations and built-in functions that have a Boolean result always
|
|
return \code{0} for false and \code{1} for true, unless otherwise
|
|
stated. (Important exception: the Boolean operations
|
|
\samp{or}\opindex{or} and \samp{and}\opindex{and} always return one of
|
|
their operands.)
|
|
|
|
\subsection{Boolean Operations}
|
|
|
|
These are the Boolean operations, ordered by ascending priority:
|
|
\indexii{Boolean}{operations}
|
|
|
|
\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
|
|
\lineiii{\var{x} or \var{y}}{if \var{x} is false, then \var{y}, else \var{x}}{(1)}
|
|
\hline
|
|
\lineiii{\var{x} and \var{y}}{if \var{x} is false, then \var{x}, else \var{y}}{(1)}
|
|
\hline
|
|
\lineiii{not \var{x}}{if \var{x} is false, then \code{1}, else \code{0}}{(2)}
|
|
\end{tableiii}
|
|
\opindex{and}
|
|
\opindex{or}
|
|
\opindex{not}
|
|
|
|
\noindent
|
|
Notes:
|
|
|
|
\begin{description}
|
|
|
|
\item[(1)]
|
|
These only evaluate their second argument if needed for their outcome.
|
|
|
|
\item[(2)]
|
|
\samp{not} has a lower priority than non-Boolean operators, so e.g.
|
|
\code{not a == b} is interpreted as \code{not(a == b)}, and
|
|
\code{a == not b} is a syntax error.
|
|
|
|
\end{description}
|
|
|
|
\subsection{Comparisons}
|
|
|
|
Comparison operations are supported by all objects. They all have the
|
|
same priority (which is higher than that of the Boolean operations).
|
|
Comparisons can be chained arbitrarily, e.g. \code{x < y <= z} is
|
|
equivalent to \code{x < y and y <= z}, except that \code{y} is
|
|
evaluated only once (but in both cases \code{z} is not evaluated at
|
|
all when \code{x < y} is found to be false).
|
|
\indexii{chaining}{comparisons}
|
|
|
|
This table summarizes the comparison operations:
|
|
|
|
\begin{tableiii}{|c|l|c|}{code}{Operation}{Meaning}{Notes}
|
|
\lineiii{<}{strictly less than}{}
|
|
\lineiii{<=}{less than or equal}{}
|
|
\lineiii{>}{strictly greater than}{}
|
|
\lineiii{>=}{greater than or equal}{}
|
|
\lineiii{==}{equal}{}
|
|
\lineiii{<>}{not equal}{(1)}
|
|
\lineiii{!=}{not equal}{(1)}
|
|
\lineiii{is}{object identity}{}
|
|
\lineiii{is not}{negated object identity}{}
|
|
\end{tableiii}
|
|
\indexii{operator}{comparison}
|
|
\opindex{==} % XXX *All* others have funny characters < ! >
|
|
\opindex{is}
|
|
\opindex{is not}
|
|
|
|
\noindent
|
|
Notes:
|
|
|
|
\begin{description}
|
|
|
|
\item[(1)]
|
|
\code{<>} and \code{!=} are alternate spellings for the same operator.
|
|
(I couldn't choose between \ABC{} and \C{}! :-)
|
|
\indexii{ABC@\ABC{}}{language}
|
|
\indexii{\C{}}{language}
|
|
|
|
\end{description}
|
|
|
|
Objects of different types, except different numeric types, never
|
|
compare equal; such objects are ordered consistently but arbitrarily
|
|
(so that sorting a heterogeneous array yields a consistent result).
|
|
Furthermore, some types (e.g., windows) support only a degenerate
|
|
notion of comparison where any two objects of that type are unequal.
|
|
Again, such objects are ordered arbitrarily but consistently.
|
|
\indexii{types}{numeric}
|
|
\indexii{objects}{comparing}
|
|
|
|
(Implementation note: objects of different types except numbers are
|
|
ordered by their type names; objects of the same types that don't
|
|
support proper comparison are ordered by their address.)
|
|
|
|
Two more operations with the same syntactic priority, \code{in} and
|
|
\code{not in}, are supported only by sequence types (below).
|
|
\opindex{in}
|
|
\opindex{not in}
|
|
|
|
\subsection{Numeric Types}
|
|
|
|
There are four numeric types: \dfn{plain integers}, \dfn{long integers},
|
|
\dfn{floating point numbers}, and \dfn{complex numbers}.
|
|
Plain integers (also just called \dfn{integers})
|
|
are implemented using \code{long} in \C{}, which gives them at least 32
|
|
bits of precision. Long integers have unlimited precision. Floating
|
|
point numbers are implemented using \code{double} in \C{}. All bets on
|
|
their precision are off unless you happen to know the machine you are
|
|
working with.
|
|
\indexii{numeric}{types}
|
|
\indexii{integer}{types}
|
|
\indexii{integer}{type}
|
|
\indexiii{long}{integer}{type}
|
|
\indexii{floating point}{type}
|
|
\indexii{complex number}{type}
|
|
\indexii{\C{}}{language}
|
|
|
|
Complex numbers have a real and imaginary part, which are both
|
|
implemented using \code{double} in \C{}. To extract these parts from
|
|
a complex number \code{z}, use \code{z.real} and \code{z.imag}.
|
|
|
|
Numbers are created by numeric literals or as the result of built-in
|
|
functions and operators. Unadorned integer literals (including hex
|
|
and octal numbers) yield plain integers. Integer literals with an \samp{L}
|
|
or \samp{l} suffix yield long integers
|
|
(\samp{L} is preferred because \samp{1l} looks too much like eleven!).
|
|
Numeric literals containing a decimal point or an exponent sign yield
|
|
floating point numbers. Appending \samp{j} or \samp{J} to a numeric
|
|
literal yields a complex number.
|
|
\indexii{numeric}{literals}
|
|
\indexii{integer}{literals}
|
|
\indexiii{long}{integer}{literals}
|
|
\indexii{floating point}{literals}
|
|
\indexii{complex number}{literals}
|
|
\indexii{hexadecimal}{literals}
|
|
\indexii{octal}{literals}
|
|
|
|
Python fully supports mixed arithmetic: when a binary arithmetic
|
|
operator has operands of different numeric types, the operand with the
|
|
``smaller'' type is converted to that of the other, where plain
|
|
integer is smaller than long integer is smaller than floating point is
|
|
smaller than complex.
|
|
Comparisons between numbers of mixed type use the same rule.%
|
|
\footnote{As a consequence, the list \code{[1, 2]} is considered equal
|
|
to \code{[1.0, 2.0]}, and similar for tuples.}
|
|
The functions \code{int()}, \code{long()}, \code{float()},
|
|
and \code{complex()} can be used
|
|
to coerce numbers to a specific type.
|
|
\index{arithmetic}
|
|
\bifuncindex{int}
|
|
\bifuncindex{long}
|
|
\bifuncindex{float}
|
|
\bifuncindex{complex}
|
|
|
|
All numeric types support the following operations, sorted by
|
|
ascending priority (operations in the same box have the same
|
|
priority; all numeric operations have a higher priority than
|
|
comparison operations):
|
|
|
|
\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
|
|
\lineiii{\var{x} + \var{y}}{sum of \var{x} and \var{y}}{}
|
|
\lineiii{\var{x} - \var{y}}{difference of \var{x} and \var{y}}{}
|
|
\hline
|
|
\lineiii{\var{x} * \var{y}}{product of \var{x} and \var{y}}{}
|
|
\lineiii{\var{x} / \var{y}}{quotient of \var{x} and \var{y}}{(1)}
|
|
\lineiii{\var{x} \%{} \var{y}}{remainder of \code{\var{x} / \var{y}}}{}
|
|
\hline
|
|
\lineiii{-\var{x}}{\var{x} negated}{}
|
|
\lineiii{+\var{x}}{\var{x} unchanged}{}
|
|
\hline
|
|
\lineiii{abs(\var{x})}{absolute value or magnitude of \var{x}}{}
|
|
\lineiii{int(\var{x})}{\var{x} converted to integer}{(2)}
|
|
\lineiii{long(\var{x})}{\var{x} converted to long integer}{(2)}
|
|
\lineiii{float(\var{x})}{\var{x} converted to floating point}{}
|
|
\lineiii{complex(\var{re},\var{im})}{a complex number with real part \var{re}, imaginary part \var{im}. \var{im} defaults to zero.}{}
|
|
\lineiii{divmod(\var{x}, \var{y})}{the pair \code{(\var{x} / \var{y}, \var{x} \%{} \var{y})}}{(3)}
|
|
\lineiii{pow(\var{x}, \var{y})}{\var{x} to the power \var{y}}{}
|
|
\lineiii{\var{x}**\var{y}}{\var{x} to the power \var{y}}{}
|
|
\end{tableiii}
|
|
\indexiii{operations on}{numeric}{types}
|
|
|
|
\noindent
|
|
Notes:
|
|
\begin{description}
|
|
|
|
\item[(1)]
|
|
For (plain or long) integer division, the result is an integer.
|
|
The result is always rounded towards minus infinity: 1/2 is 0,
|
|
(-1)/2 is -1, 1/(-2) is -1, and (-1)/(-2) is 0.
|
|
\indexii{integer}{division}
|
|
\indexiii{long}{integer}{division}
|
|
|
|
\item[(2)]
|
|
Conversion from floating point to (long or plain) integer may round or
|
|
truncate as in \C{}; see functions \code{floor()} and \code{ceil()} in
|
|
module \code{math} for well-defined conversions.
|
|
\bifuncindex{floor}
|
|
\bifuncindex{ceil}
|
|
\indexii{numeric}{conversions}
|
|
\refbimodindex{math}
|
|
\indexii{\C{}}{language}
|
|
|
|
\item[(3)]
|
|
See the section on built-in functions for an exact definition.
|
|
|
|
\end{description}
|
|
% XXXJH exceptions: overflow (when? what operations?) zerodivision
|
|
|
|
\subsubsection{Bit-string Operations on Integer Types}
|
|
\nodename{Bit-string Operations}
|
|
|
|
Plain and long integer types support additional operations that make
|
|
sense only for bit-strings. Negative numbers are treated as their 2's
|
|
complement value (for long integers, this assumes a sufficiently large
|
|
number of bits that no overflow occurs during the operation).
|
|
|
|
The priorities of the binary bit-wise operations are all lower than
|
|
the numeric operations and higher than the comparisons; the unary
|
|
operation \samp{\~} has the same priority as the other unary numeric
|
|
operations (\samp{+} and \samp{-}).
|
|
|
|
This table lists the bit-string operations sorted in ascending
|
|
priority (operations in the same box have the same priority):
|
|
|
|
\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
|
|
\lineiii{\var{x} | \var{y}}{bitwise \dfn{or} of \var{x} and \var{y}}{}
|
|
\hline
|
|
\lineiii{\var{x} \^{} \var{y}}{bitwise \dfn{exclusive or} of \var{x} and \var{y}}{}
|
|
\hline
|
|
\lineiii{\var{x} \&{} \var{y}}{bitwise \dfn{and} of \var{x} and \var{y}}{}
|
|
\hline
|
|
\lineiii{\var{x} << \var{n}}{\var{x} shifted left by \var{n} bits}{(1), (2)}
|
|
\lineiii{\var{x} >> \var{n}}{\var{x} shifted right by \var{n} bits}{(1), (3)}
|
|
\hline
|
|
\hline
|
|
\lineiii{\~\var{x}}{the bits of \var{x} inverted}{}
|
|
\end{tableiii}
|
|
\indexiii{operations on}{integer}{types}
|
|
\indexii{bit-string}{operations}
|
|
\indexii{shifting}{operations}
|
|
\indexii{masking}{operations}
|
|
|
|
\noindent
|
|
Notes:
|
|
\begin{description}
|
|
\item[(1)] Negative shift counts are illegal and cause a
|
|
\exception{ValueError} to be raised.
|
|
\item[(2)] A left shift by \var{n} bits is equivalent to
|
|
multiplication by \code{pow(2, \var{n})} without overflow check.
|
|
\item[(3)] A right shift by \var{n} bits is equivalent to
|
|
division by \code{pow(2, \var{n})} without overflow check.
|
|
\end{description}
|
|
|
|
\subsection{Sequence Types}
|
|
|
|
There are three sequence types: strings, lists and tuples.
|
|
|
|
Strings literals are written in single or double quotes:
|
|
\code{'xyzzy'}, \code{"frobozz"}. See Chapter 2 of the \emph{Python
|
|
Reference Manual} for more about string literals. Lists are
|
|
constructed with square brackets, separating items with commas:
|
|
\code{[a, b, c]}. Tuples are constructed by the comma operator (not
|
|
within square brackets), with or without enclosing parentheses, but an
|
|
empty tuple must have the enclosing parentheses, e.g.,
|
|
\code{a, b, c} or \code{()}. A single item tuple must have a trailing
|
|
comma, e.g., \code{(d,)}.
|
|
\indexii{sequence}{types}
|
|
\indexii{string}{type}
|
|
\indexii{tuple}{type}
|
|
\indexii{list}{type}
|
|
|
|
Sequence types support the following operations. The \samp{in} and
|
|
\samp{not in} operations have the same priorities as the comparison
|
|
operations. The \samp{+} and \samp{*} operations have the same
|
|
priority as the corresponding numeric operations.\footnote{They must
|
|
have since the parser can't tell the type of the operands.}
|
|
|
|
This table lists the sequence operations sorted in ascending priority
|
|
(operations in the same box have the same priority). In the table,
|
|
\var{s} and \var{t} are sequences of the same type; \var{n}, \var{i}
|
|
and \var{j} are integers:
|
|
|
|
\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
|
|
\lineiii{\var{x} in \var{s}}{\code{1} if an item of \var{s} is equal to \var{x}, else \code{0}}{}
|
|
\lineiii{\var{x} not in \var{s}}{\code{0} if an item of \var{s} is
|
|
equal to \var{x}, else \code{1}}{}
|
|
\hline
|
|
\lineiii{\var{s} + \var{t}}{the concatenation of \var{s} and \var{t}}{}
|
|
\hline
|
|
\lineiii{\var{s} * \var{n}{\rm ,} \var{n} * \var{s}}{\var{n} copies of \var{s} concatenated}{(3)}
|
|
\hline
|
|
\lineiii{\var{s}[\var{i}]}{\var{i}'th item of \var{s}, origin 0}{(1)}
|
|
\lineiii{\var{s}[\var{i}:\var{j}]}{slice of \var{s} from \var{i} to \var{j}}{(1), (2)}
|
|
\hline
|
|
\lineiii{len(\var{s})}{length of \var{s}}{}
|
|
\lineiii{min(\var{s})}{smallest item of \var{s}}{}
|
|
\lineiii{max(\var{s})}{largest item of \var{s}}{}
|
|
\end{tableiii}
|
|
\indexiii{operations on}{sequence}{types}
|
|
\bifuncindex{len}
|
|
\bifuncindex{min}
|
|
\bifuncindex{max}
|
|
\indexii{concatenation}{operation}
|
|
\indexii{repetition}{operation}
|
|
\indexii{subscript}{operation}
|
|
\indexii{slice}{operation}
|
|
\opindex{in}
|
|
\opindex{not in}
|
|
|
|
\noindent
|
|
Notes:
|
|
|
|
\begin{description}
|
|
|
|
\item[(1)] If \var{i} or \var{j} is negative, the index is relative to
|
|
the end of the string, i.e., \code{len(\var{s}) + \var{i}} or
|
|
\code{len(\var{s}) + \var{j}} is substituted. But note that \code{-0} is
|
|
still \code{0}.
|
|
|
|
\item[(2)] The slice of \var{s} from \var{i} to \var{j} is defined as
|
|
the sequence of items with index \var{k} such that \code{\var{i} <=
|
|
\var{k} < \var{j}}. If \var{i} or \var{j} is greater than
|
|
\code{len(\var{s})}, use \code{len(\var{s})}. If \var{i} is omitted,
|
|
use \code{0}. If \var{j} is omitted, use \code{len(\var{s})}. If
|
|
\var{i} is greater than or equal to \var{j}, the slice is empty.
|
|
|
|
\item[(3)] Values of \var{n} less than \code{0} are treated as
|
|
\code{0} (which yields an empty sequence of the same type as
|
|
\var{s}).
|
|
|
|
\end{description}
|
|
|
|
\subsubsection{More String Operations}
|
|
|
|
String objects have one unique built-in operation: the \code{\%}
|
|
operator (modulo) with a string left argument interprets this string
|
|
as a \C{} \cfunction{sprintf()} format string to be applied to the
|
|
right argument, and returns the string resulting from this formatting
|
|
operation.
|
|
|
|
The right argument should be a tuple with one item for each argument
|
|
required by the format string; if the string requires a single
|
|
argument, the right argument may also be a single non-tuple object.%
|
|
\footnote{A tuple object in this case should be a singleton.}
|
|
The following format characters are understood:
|
|
\%, c, s, i, d, u, o, x, X, e, E, f, g, G.
|
|
Width and precision may be a * to specify that an integer argument
|
|
specifies the actual width or precision. The flag characters -, +,
|
|
blank, \# and 0 are understood. The size specifiers h, l or L may be
|
|
present but are ignored. The \code{\%s} conversion takes any Python
|
|
object and converts it to a string using \code{str()} before
|
|
formatting it. The ANSI features \code{\%p} and \code{\%n}
|
|
are not supported. Since Python strings have an explicit length,
|
|
\code{\%s} conversions don't assume that \code{'\e0'} is the end of
|
|
the string.
|
|
|
|
For safety reasons, floating point precisions are clipped to 50;
|
|
\code{\%f} conversions for numbers whose absolute value is over 1e25
|
|
are replaced by \code{\%g} conversions.%
|
|
\footnote{These numbers are fairly arbitrary. They are intended to
|
|
avoid printing endless strings of meaningless digits without hampering
|
|
correct use and without having to know the exact precision of floating
|
|
point values on a particular machine.}
|
|
All other errors raise exceptions.
|
|
|
|
If the right argument is a dictionary (or any kind of mapping), then
|
|
the formats in the string must have a parenthesized key into that
|
|
dictionary inserted immediately after the \code{\%} character, and
|
|
each format formats the corresponding entry from the mapping. E.g.
|
|
\begin{verbatim}
|
|
>>> count = 2
|
|
>>> language = 'Python'
|
|
>>> print '%(language)s has %(count)03d quote types.' % vars()
|
|
Python has 002 quote types.
|
|
>>>
|
|
\end{verbatim}
|
|
In this case no * specifiers may occur in a format (since they
|
|
require a sequential parameter list).
|
|
|
|
Additional string operations are defined in standard module
|
|
\code{string} and in built-in module \code{re}.
|
|
\refstmodindex{string}
|
|
\refbimodindex{re}
|
|
|
|
\subsubsection{Mutable Sequence Types}
|
|
|
|
List objects support additional operations that allow in-place
|
|
modification of the object.
|
|
These operations would be supported by other mutable sequence types
|
|
(when added to the language) as well.
|
|
Strings and tuples are immutable sequence types and such objects cannot
|
|
be modified once created.
|
|
The following operations are defined on mutable sequence types (where
|
|
\var{x} is an arbitrary object):
|
|
\indexiii{mutable}{sequence}{types}
|
|
\indexii{list}{type}
|
|
|
|
\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
|
|
\lineiii{\var{s}[\var{i}] = \var{x}}
|
|
{item \var{i} of \var{s} is replaced by \var{x}}{}
|
|
\lineiii{\var{s}[\var{i}:\var{j}] = \var{t}}
|
|
{slice of \var{s} from \var{i} to \var{j} is replaced by \var{t}}{}
|
|
\lineiii{del \var{s}[\var{i}:\var{j}]}
|
|
{same as \code{\var{s}[\var{i}:\var{j}] = []}}{}
|
|
\lineiii{\var{s}.append(\var{x})}
|
|
{same as \code{\var{s}[len(\var{s}):len(\var{s})] = [\var{x}]}}{}
|
|
\lineiii{\var{s}.count(\var{x})}
|
|
{return number of \var{i}'s for which \code{\var{s}[\var{i}] == \var{x}}}{}
|
|
\lineiii{\var{s}.index(\var{x})}
|
|
{return smallest \var{i} such that \code{\var{s}[\var{i}] == \var{x}}}{(1)}
|
|
\lineiii{\var{s}.insert(\var{i}, \var{x})}
|
|
{same as \code{\var{s}[\var{i}:\var{i}] = [\var{x}]}
|
|
if \code{\var{i} >= 0}}{}
|
|
\lineiii{\var{s}.remove(\var{x})}
|
|
{same as \code{del \var{s}[\var{s}.index(\var{x})]}}{(1)}
|
|
\lineiii{\var{s}.reverse()}
|
|
{reverses the items of \var{s} in place}{(3)}
|
|
\lineiii{\var{s}.sort()}
|
|
{sort the items of \var{s} in place}{(2), (3)}
|
|
\end{tableiii}
|
|
\indexiv{operations on}{mutable}{sequence}{types}
|
|
\indexiii{operations on}{sequence}{types}
|
|
\indexiii{operations on}{list}{type}
|
|
\indexii{subscript}{assignment}
|
|
\indexii{slice}{assignment}
|
|
\stindex{del}
|
|
\setindexsubitem{(list method)}
|
|
\ttindex{append}
|
|
\ttindex{count}
|
|
\ttindex{index}
|
|
\ttindex{insert}
|
|
\ttindex{remove}
|
|
\ttindex{reverse}
|
|
\ttindex{sort}
|
|
|
|
\noindent
|
|
Notes:
|
|
\begin{description}
|
|
\item[(1)] Raises an exception when \var{x} is not found in \var{s}.
|
|
|
|
\item[(2)] The \code{sort()} method takes an optional argument
|
|
specifying a comparison function of two arguments (list items) which
|
|
should return \code{-1}, \code{0} or \code{1} depending on whether the
|
|
first argument is considered smaller than, equal to, or larger than the
|
|
second argument. Note that this slows the sorting process down
|
|
considerably; e.g. to sort a list in reverse order it is much faster
|
|
to use calls to \code{sort()} and \code{reverse()} than to use
|
|
\code{sort()} with a comparison function that reverses the ordering of
|
|
the elements.
|
|
|
|
\item[(3)] The \code{sort()} and \code{reverse()} methods modify the
|
|
list in place for economy of space when sorting or reversing a large
|
|
list. They don't return the sorted or reversed list to remind you of
|
|
this side effect.
|
|
|
|
\end{description}
|
|
|
|
\subsection{Mapping Types}
|
|
|
|
A \dfn{mapping} object maps values of one type (the key type) to
|
|
arbitrary objects. Mappings are mutable objects. There is currently
|
|
only one standard mapping type, the \dfn{dictionary}. A dictionary's keys are
|
|
almost arbitrary values. The only types of values not acceptable as
|
|
keys are values containing lists or dictionaries or other mutable
|
|
types that are compared by value rather than by object identity.
|
|
Numeric types used for keys obey the normal rules for numeric
|
|
comparison: if two numbers compare equal (e.g. \code{1} and
|
|
\code{1.0}) then they can be used interchangeably to index the same
|
|
dictionary entry.
|
|
|
|
\indexii{mapping}{types}
|
|
\indexii{dictionary}{type}
|
|
|
|
Dictionaries are created by placing a comma-separated list of
|
|
\code{\var{key}:\,\var{value}} pairs within braces, for example:
|
|
\code{\{'jack':\,4098, 'sjoerd':\,4127\}} or
|
|
\code{\{4098:\,'jack', 4127:\,'sjoerd'\}}.
|
|
|
|
The following operations are defined on mappings (where \var{a} is a
|
|
mapping, \var{k} is a key and \var{x} is an arbitrary object):
|
|
|
|
\begin{tableiii}{|c|l|c|}{code}{Operation}{Result}{Notes}
|
|
\lineiii{len(\var{a})}{the number of items in \var{a}}{}
|
|
\lineiii{\var{a}[\var{k}]}{the item of \var{a} with key \var{k}}{(1)}
|
|
\lineiii{\var{a}[\var{k}] = \var{x}}{set \code{\var{a}[\var{k}]} to \var{x}}{}
|
|
\lineiii{del \var{a}[\var{k}]}{remove \code{\var{a}[\var{k}]} from \var{a}}{(1)}
|
|
\lineiii{\var{a}.clear()}{remove all items from \code{a}}{}
|
|
\lineiii{\var{a}.copy()}{a (shallow) copy of \code{a}}{}
|
|
\lineiii{\var{a}.has_key(\var{k})}{\code{1} if \var{a} has a key \var{k}, else \code{0}}{}
|
|
\lineiii{\var{a}.items()}{a copy of \var{a}'s list of (key, item) pairs}{(2)}
|
|
\lineiii{\var{a}.keys()}{a copy of \var{a}'s list of keys}{(2)}
|
|
\lineiii{\var{a}.update(\var{b})}{\code{for k, v in \var{b}.items(): \var{a}[k] = v}}{(3)}
|
|
\lineiii{\var{a}.values()}{a copy of \var{a}'s list of values}{(2)}
|
|
\lineiii{\var{a}.get(\var{k}, \var{f})}{the item of \var{a} with key \var{k}}{(4)}
|
|
\end{tableiii}
|
|
\indexiii{operations on}{mapping}{types}
|
|
\indexiii{operations on}{dictionary}{type}
|
|
\stindex{del}
|
|
\bifuncindex{len}
|
|
\setindexsubitem{(dictionary method)}
|
|
\ttindex{keys}
|
|
\ttindex{has_key}
|
|
|
|
\noindent
|
|
Notes:
|
|
\begin{description}
|
|
\item[(1)] Raises an exception if \var{k} is not in the map.
|
|
|
|
\item[(2)] Keys and values are listed in random order.
|
|
|
|
\item[(3)] \var{b} must be of the same type as \var{a}.
|
|
|
|
\item[(4)] Never raises an exception if \var{k} is not in the map,
|
|
instead it returns \var{f}. \var{f} is optional, when not provided
|
|
and \var{k} is not in the map, \code{None} is returned.
|
|
\end{description}
|
|
|
|
\subsection{Other Built-in Types}
|
|
|
|
The interpreter supports several other kinds of objects.
|
|
Most of these support only one or two operations.
|
|
|
|
\subsubsection{Modules}
|
|
|
|
The only special operation on a module is attribute access:
|
|
\code{\var{m}.\var{name}}, where \var{m} is a module and \var{name} accesses
|
|
a name defined in \var{m}'s symbol table. Module attributes can be
|
|
assigned to. (Note that the \code{import} statement is not, strictly
|
|
spoken, an operation on a module object; \code{import \var{foo}} does not
|
|
require a module object named \var{foo} to exist, rather it requires
|
|
an (external) \emph{definition} for a module named \var{foo}
|
|
somewhere.)
|
|
|
|
A special member of every module is \code{__dict__}.
|
|
This is the dictionary containing the module's symbol table.
|
|
Modifying this dictionary will actually change the module's symbol
|
|
table, but direct assignment to the \code{__dict__} attribute is not
|
|
possible (i.e., you can write \code{\var{m}.__dict__['a'] = 1}, which
|
|
defines \code{\var{m}.a} to be \code{1}, but you can't write \code{\var{m}.__dict__ = \{\}}.
|
|
|
|
Modules are written like this: \code{<module 'sys'>}.
|
|
|
|
\subsubsection{Classes and Class Instances}
|
|
\nodename{Classes and Instances}
|
|
|
|
See Chapters 3 and 7 of the \emph{Python Reference Manual} for these.
|
|
|
|
\subsubsection{Functions}
|
|
|
|
Function objects are created by function definitions. The only
|
|
operation on a function object is to call it:
|
|
\code{\var{func}(\var{argument-list})}.
|
|
|
|
There are really two flavors of function objects: built-in functions
|
|
and user-defined functions. Both support the same operation (to call
|
|
the function), but the implementation is different, hence the
|
|
different object types.
|
|
|
|
The implementation adds two special read-only attributes:
|
|
\code{\var{f}.func_code} is a function's \dfn{code object} (see below) and
|
|
\code{\var{f}.func_globals} is the dictionary used as the function's
|
|
global name space (this is the same as \code{\var{m}.__dict__} where
|
|
\var{m} is the module in which the function \var{f} was defined).
|
|
|
|
\subsubsection{Methods}
|
|
\obindex{method}
|
|
|
|
Methods are functions that are called using the attribute notation.
|
|
There are two flavors: built-in methods (such as \code{append()} on
|
|
lists) and class instance methods. Built-in methods are described
|
|
with the types that support them.
|
|
|
|
The implementation adds two special read-only attributes to class
|
|
instance methods: \code{\var{m}.im_self} is the object whose method this
|
|
is, and \code{\var{m}.im_func} is the function implementing the method.
|
|
Calling \code{\var{m}(\var{arg-1}, \var{arg-2}, {\rm \ldots},
|
|
\var{arg-n})} is completely equivalent to calling
|
|
\code{\var{m}.im_func(\var{m}.im_self, \var{arg-1}, \var{arg-2}, {\rm
|
|
\ldots}, \var{arg-n})}.
|
|
|
|
See the \emph{Python Reference Manual} for more information.
|
|
|
|
\subsubsection{Code Objects}
|
|
\obindex{code}
|
|
|
|
Code objects are used by the implementation to represent
|
|
``pseudo-compiled'' executable Python code such as a function body.
|
|
They differ from function objects because they don't contain a
|
|
reference to their global execution environment. Code objects are
|
|
returned by the built-in \code{compile()} function and can be
|
|
extracted from function objects through their \code{func_code}
|
|
attribute.
|
|
\bifuncindex{compile}
|
|
\ttindex{func_code}
|
|
|
|
A code object can be executed or evaluated by passing it (instead of a
|
|
source string) to the \code{exec} statement or the built-in
|
|
\code{eval()} function.
|
|
\stindex{exec}
|
|
\bifuncindex{eval}
|
|
|
|
See the \emph{Python Reference Manual} for more information.
|
|
|
|
\subsubsection{Type Objects}
|
|
|
|
Type objects represent the various object types. An object's type is
|
|
accessed by the built-in function \code{type()}. There are no special
|
|
operations on types. The standard module \code{types} defines names
|
|
for all standard built-in types.
|
|
\bifuncindex{type}
|
|
\refstmodindex{types}
|
|
|
|
Types are written like this: \code{<type 'int'>}.
|
|
|
|
\subsubsection{The Null Object}
|
|
|
|
This object is returned by functions that don't explicitly return a
|
|
value. It supports no special operations. There is exactly one null
|
|
object, named \code{None} (a built-in name).
|
|
|
|
It is written as \code{None}.
|
|
|
|
\subsubsection{File Objects}
|
|
|
|
File objects are implemented using \C{}'s \code{stdio} package and can be
|
|
created with the built-in function \code{open()} described under
|
|
Built-in Functions below. They are also returned by some other
|
|
built-in functions and methods, e.g.\ \code{posix.popen()} and
|
|
\code{posix.fdopen()} and the \code{makefile()} method of socket
|
|
objects.
|
|
\bifuncindex{open}
|
|
\refbimodindex{posix}
|
|
\refbimodindex{socket}
|
|
|
|
When a file operation fails for an I/O-related reason, the exception
|
|
\code{IOError} is raised. This includes situations where the
|
|
operation is not defined for some reason, like \code{seek()} on a tty
|
|
device or writing a file opened for reading.
|
|
|
|
Files have the following methods:
|
|
|
|
|
|
\setindexsubitem{(file method)}
|
|
|
|
\begin{funcdesc}{close}{}
|
|
Close the file. A closed file cannot be read or written anymore.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{flush}{}
|
|
Flush the internal buffer, like \code{stdio}'s \code{fflush()}.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{isatty}{}
|
|
Return \code{1} if the file is connected to a tty(-like) device, else
|
|
\code{0}.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{fileno}{}
|
|
Return the integer ``file descriptor'' that is used by the underlying
|
|
implementation to request I/O operations from the operating system.
|
|
This can be useful for other, lower level interfaces that use file
|
|
descriptors, e.g. module \code{fcntl} or \code{os.read()} and friends.
|
|
\refbimodindex{fcntl}
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{read}{\optional{size}}
|
|
Read at most \var{size} bytes from the file (less if the read hits
|
|
\EOF{} or no more data is immediately available on a pipe, tty or
|
|
similar device). If the \var{size} argument is negative or omitted,
|
|
read all data until \EOF{} is reached. The bytes are returned as a string
|
|
object. An empty string is returned when \EOF{} is encountered
|
|
immediately. (For certain files, like ttys, it makes sense to
|
|
continue reading after an \EOF{} is hit.)
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{readline}{\optional{size}}
|
|
Read one entire line from the file. A trailing newline character is
|
|
kept in the string%
|
|
\footnote{The advantage of leaving the newline on is that an empty string
|
|
can be returned to mean \EOF{} without being ambiguous. Another
|
|
advantage is that (in cases where it might matter, e.g. if you
|
|
want to make an exact copy of a file while scanning its lines)
|
|
you can tell whether the last line of a file ended in a newline
|
|
or not (yes this happens!).}
|
|
(but may be absent when a file ends with an
|
|
incomplete line). If the \var{size} argument is present and
|
|
non-negative, it is a maximum byte count (including the trailing
|
|
newline) and an incomplete line may be returned.
|
|
An empty string is returned when \EOF{} is hit
|
|
immediately. Note: unlike \code{stdio}'s \code{fgets()}, the returned
|
|
string contains null characters (\code{'\e 0'}) if they occurred in the
|
|
input.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{readlines}{\optional{sizehint}}
|
|
Read until \EOF{} using \code{readline()} and return a list containing
|
|
the lines thus read. If the optional \var{sizehint} argument is
|
|
present, instead of reading up to \EOF{}, whole lines totalling
|
|
approximately \var{sizehint} bytes (possibly after rounding up to an
|
|
internal buffer size) are read.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{seek}{offset\, whence}
|
|
Set the file's current position, like \code{stdio}'s \code{fseek()}.
|
|
The \var{whence} argument is optional and defaults to \code{0}
|
|
(absolute file positioning); other values are \code{1} (seek
|
|
relative to the current position) and \code{2} (seek relative to the
|
|
file's end). There is no return value.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{tell}{}
|
|
Return the file's current position, like \code{stdio}'s \code{ftell()}.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{truncate}{\optional{size}}
|
|
Truncate the file's size. If the optional size argument present, the
|
|
file is truncated to (at most) that size. The size defaults to the
|
|
current position. Availability of this function depends on the
|
|
operating system version (e.g., not all \UNIX{} versions support this
|
|
operation).
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{write}{str}
|
|
Write a string to the file. There is no return value. Note: due to
|
|
buffering, the string may not actually show up in the file until
|
|
the \code{flush()} or \code{close()} method is called.
|
|
\end{funcdesc}
|
|
|
|
\begin{funcdesc}{writelines}{list}
|
|
Write a list of strings to the file. There is no return value.
|
|
(The name is intended to match \code{readlines}; \code{writelines}
|
|
does not add line separators.)
|
|
\end{funcdesc}
|
|
|
|
File objects also offer the following attributes:
|
|
|
|
\begin{datadesc}{closed}
|
|
Boolean indicating the current state of the file object. This is a
|
|
read-only attribute; the \method{close()} method changes the value.
|
|
\end{datadesc}
|
|
|
|
\begin{datadesc}{mode}
|
|
The I/O mode for the file. If the file was created using the
|
|
\function{open()} built-in function, this will be the value of the
|
|
\var{mode} parameter. This is a read-only attribute.
|
|
\end{datadesc}
|
|
|
|
\begin{datadesc}{name}
|
|
If the file object was created using \function{open()}, the name of
|
|
the file. Otherwise, some string that indicates the source of the
|
|
file object, of the form \samp{<\mbox{\ldots}>}. This is a read-only
|
|
attribute.
|
|
\end{datadesc}
|
|
|
|
\begin{datadesc}{softspace}
|
|
Boolean that indicates whether a space character needs to be printed
|
|
before another value when using the \keyword{print} statement.
|
|
Classes that are trying to simulate a file object should also have a
|
|
writable \code{softspace} attribute, which should be initialized to
|
|
zero. This will be automatic for classes implemented in Python; types
|
|
implemented in \C{} will have to provide a writable \code{softspace}
|
|
attribute.
|
|
\end{datadesc}
|
|
|
|
\subsubsection{Internal Objects}
|
|
|
|
See the \emph{Python Reference Manual} for this information. It
|
|
describes code objects, stack frame objects, traceback objects, and
|
|
slice objects.
|
|
|
|
\subsection{Special Attributes}
|
|
|
|
The implementation adds a few special read-only attributes to several
|
|
object types, where they are relevant:
|
|
|
|
\begin{itemize}
|
|
|
|
\item
|
|
\code{\var{x}.__dict__} is a dictionary of some sort used to store an
|
|
object's (writable) attributes;
|
|
|
|
\item
|
|
\code{\var{x}.__methods__} lists the methods of many built-in object types,
|
|
e.g., \code{[].__methods__} yields
|
|
\code{['append', 'count', 'index', 'insert', 'remove', 'reverse', 'sort']};
|
|
|
|
\item
|
|
\code{\var{x}.__members__} lists data attributes;
|
|
|
|
\item
|
|
\code{\var{x}.__class__} is the class to which a class instance belongs;
|
|
|
|
\item
|
|
\code{\var{x}.__bases__} is the tuple of base classes of a class object.
|
|
|
|
\end{itemize}
|