I/O and Network
General I/O
Base.stdout
— Constantstdout
Global variable referring to the standard out stream.
Base.stderr
— Constantstderr
Global variable referring to the standard error stream.
Base.stdin
— Constantstdin
Global variable referring to the standard input stream.
Base.open
— Functionopen(f::Function, args...; kwargs...)
Apply the function f
to the result of open(args...; kwargs...)
and close the resulting file descriptor upon completion.
Examples
julia> open("myfile.txt", "w") do io
write(io, "Hello world!")
end;
julia> open(f->read(f, String), "myfile.txt")
"Hello world!"
julia> rm("myfile.txt")
open(filename::AbstractString; lock = true, keywords...) -> IOStream
Open a file in a mode specified by five boolean keyword arguments:
Keyword | Description | Default |
---|---|---|
read | open for reading | !write |
write | open for writing | truncate | append |
create | create if non-existent | !read & write | truncate | append |
truncate | truncate to zero size | !read & write |
append | seek to end | false |
The default when no keywords are passed is to open files for reading only. Returns a stream for accessing the opened file.
The lock
keyword argument controls whether operations will be locked for safe multi-threaded access.
The lock
argument is available as of Julia 1.5.
open(filename::AbstractString, [mode::AbstractString]; lock = true) -> IOStream
Alternate syntax for open, where a string-based mode specifier is used instead of the five booleans. The values of mode
correspond to those from fopen(3)
or Perl open
, and are equivalent to setting the following boolean groups:
Mode | Description | Keywords |
---|---|---|
r | read | none |
w | write, create, truncate | write = true |
a | write, create, append | append = true |
r+ | read, write | read = true, write = true |
w+ | read, write, create, truncate | truncate = true, read = true |
a+ | read, write, create, append | append = true, read = true |
The lock
keyword argument controls whether operations will be locked for safe multi-threaded access.
Examples
julia> io = open("myfile.txt", "w");
julia> write(io, "Hello world!");
julia> close(io);
julia> io = open("myfile.txt", "r");
julia> read(io, String)
"Hello world!"
julia> write(io, "This file is read only")
ERROR: ArgumentError: write failed, IOStream is not writeable
[...]
julia> close(io)
julia> io = open("myfile.txt", "a");
julia> write(io, "This stream is not read only")
28
julia> close(io)
julia> rm("myfile.txt")
The lock
argument is available as of Julia 1.5.
open(fd::OS_HANDLE) -> IO
Take a raw file descriptor wrap it in a Julia-aware IO type, and take ownership of the fd handle. Call open(Libc.dup(fd))
to avoid the ownership capture of the original handle.
Do not call this on a handle that's already owned by some other part of the system.
open(command, mode::AbstractString, stdio=devnull)
Run command
asynchronously. Like open(command, stdio; read, write)
except specifying the read and write flags via a mode string instead of keyword arguments. Possible mode strings are:
Mode | Description | Keywords |
---|---|---|
r | read | none |
w | write | write = true |
r+ | read, write | read = true, write = true |
w+ | read, write | read = true, write = true |
open(command, stdio=devnull; write::Bool = false, read::Bool = !write)
Start running command
asynchronously, and return a process::IO
object. If read
is true, then reads from the process come from the process's standard output and stdio
optionally specifies the process's standard input stream. If write
is true, then writes go to the process's standard input and stdio
optionally specifies the process's standard output stream. The process's standard error stream is connected to the current global stderr
.
open(f::Function, command, args...; kwargs...)
Similar to open(command, args...; kwargs...)
, but calls f(stream)
on the resulting process stream, then closes the input stream and waits for the process to complete. Return the value returned by f
on success. Throw an error if the process failed, or if the process attempts to print anything to stdout.
Base.IOStream
— TypeIOStream
A buffered IO stream wrapping an OS file descriptor. Mostly used to represent files returned by open
.
Base.IOBuffer
— TypeIOBuffer([data::AbstractVector{UInt8}]; keywords...) -> IOBuffer
Create an in-memory I/O stream, which may optionally operate on a pre-existing array.
It may take optional keyword arguments:
read
,write
,append
: restricts operations to the buffer; seeopen
for details.truncate
: truncates the buffer size to zero length.maxsize
: specifies a size beyond which the buffer may not be grown.sizehint
: suggests a capacity of the buffer (data
must implementsizehint!(data, size)
).
When data
is not given, the buffer will be both readable and writable by default.
Examples
julia> io = IOBuffer();
julia> write(io, "JuliaLang is a GitHub organization.", " It has many members.")
56
julia> String(take!(io))
"JuliaLang is a GitHub organization. It has many members."
julia> io = IOBuffer(b"JuliaLang is a GitHub organization.")
IOBuffer(data=UInt8[...], readable=true, writable=false, seekable=true, append=false, size=35, maxsize=Inf, ptr=1, mark=-1)
julia> read(io, String)
"JuliaLang is a GitHub organization."
julia> write(io, "This isn't writable.")
ERROR: ArgumentError: ensureroom failed, IOBuffer is not writeable
julia> io = IOBuffer(UInt8[], read=true, write=true, maxsize=34)
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=34, ptr=1, mark=-1)
julia> write(io, "JuliaLang is a GitHub organization.")
34
julia> String(take!(io))
"JuliaLang is a GitHub organization"
julia> length(read(IOBuffer(b"data", read=true, truncate=false)))
4
julia> length(read(IOBuffer(b"data", read=true, truncate=true)))
0
IOBuffer(string::String)
Create a read-only IOBuffer
on the data underlying the given string.
Examples
julia> io = IOBuffer("Haho");
julia> String(take!(io))
"Haho"
julia> String(take!(io))
"Haho"
Base.take!
— Methodtake!(b::IOBuffer)
Obtain the contents of an IOBuffer
as an array. Afterwards, the IOBuffer
is reset to its initial state.
Examples
julia> io = IOBuffer();
julia> write(io, "JuliaLang is a GitHub organization.", " It has many members.")
56
julia> String(take!(io))
"JuliaLang is a GitHub organization. It has many members."
Base.fdio
— Functionfdio([name::AbstractString, ]fd::Integer[, own::Bool=false]) -> IOStream
Create an IOStream
object from an integer file descriptor. If own
is true
, closing this object will close the underlying descriptor. By default, an IOStream
is closed when it is garbage collected. name
allows you to associate the descriptor with a named file.
Base.flush
— Functionflush(stream)
Commit all currently buffered writes to the given stream.
Base.close
— Functionclose(stream)
Close an I/O stream. Performs a flush
first.
Base.write
— Functionwrite(io::IO, x)
write(filename::AbstractString, x)
Write the canonical binary representation of a value to the given I/O stream or file. Return the number of bytes written into the stream. See also print
to write a text representation (with an encoding that may depend upon io
).
The endianness of the written value depends on the endianness of the host system. Convert to/from a fixed endianness when writing/reading (e.g. using htol
and ltoh
) to get results that are consistent across platforms.
You can write multiple values with the same write
call. i.e. the following are equivalent:
write(io, x, y...)
write(io, x) + write(io, y...)
Examples
Consistent serialization:
julia> fname = tempname(); # random temporary filename
julia> open(fname,"w") do f
# Make sure we write 64bit integer in little-endian byte order
write(f,htol(Int64(42)))
end
8
julia> open(fname,"r") do f
# Convert back to host byte order and host integer type
Int(ltoh(read(f,Int64)))
end
42
Merging write calls:
julia> io = IOBuffer();
julia> write(io, "JuliaLang is a GitHub organization.", " It has many members.")
56
julia> String(take!(io))
"JuliaLang is a GitHub organization. It has many members."
julia> write(io, "Sometimes those members") + write(io, " write documentation.")
44
julia> String(take!(io))
"Sometimes those members write documentation."
User-defined plain-data types without write
methods can be written when wrapped in a Ref
:
julia> struct MyStruct; x::Float64; end
julia> io = IOBuffer()
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=0, maxsize=Inf, ptr=1, mark=-1)
julia> write(io, Ref(MyStruct(42.0)))
8
julia> seekstart(io); read!(io, Ref(MyStruct(NaN)))
Base.RefValue{MyStruct}(MyStruct(42.0))
Base.read
— Functionread(io::IO, T)
Read a single value of type T
from io
, in canonical binary representation.
Note that Julia does not convert the endianness for you. Use ntoh
or ltoh
for this purpose.
read(io::IO, String)
Read the entirety of io
, as a String
(see also readchomp
).
Examples
julia> io = IOBuffer("JuliaLang is a GitHub organization");
julia> read(io, Char)
'J': ASCII/Unicode U+004A (category Lu: Letter, uppercase)
julia> io = IOBuffer("JuliaLang is a GitHub organization");
julia> read(io, String)
"JuliaLang is a GitHub organization"
read(filename::AbstractString, args...)
Open a file and read its contents. args
is passed to read
: this is equivalent to open(io->read(io, args...), filename)
.
read(filename::AbstractString, String)
Read the entire contents of a file as a string.
read(s::IO, nb=typemax(Int))
Read at most nb
bytes from s
, returning a Vector{UInt8}
of the bytes read.
read(s::IOStream, nb::Integer; all=true)
Read at most nb
bytes from s
, returning a Vector{UInt8}
of the bytes read.
If all
is true
(the default), this function will block repeatedly trying to read all requested bytes, until an error or end-of-file occurs. If all
is false
, at most one read
call is performed, and the amount of data returned is device-dependent. Note that not all stream types support the all
option.
read(command::Cmd)
Run command
and return the resulting output as an array of bytes.
read(command::Cmd, String)
Run command
and return the resulting output as a String
.
Base.read!
— Functionread!(stream::IO, array::AbstractArray)
read!(filename::AbstractString, array::AbstractArray)
Read binary data from an I/O stream or file, filling in array
.
Base.readbytes!
— Functionreadbytes!(stream::IO, b::AbstractVector{UInt8}, nb=length(b))
Read at most nb
bytes from stream
into b
, returning the number of bytes read. The size of b
will be increased if needed (i.e. if nb
is greater than length(b)
and enough bytes could be read), but it will never be decreased.
readbytes!(stream::IOStream, b::AbstractVector{UInt8}, nb=length(b); all::Bool=true)
Read at most nb
bytes from stream
into b
, returning the number of bytes read. The size of b
will be increased if needed (i.e. if nb
is greater than length(b)
and enough bytes could be read), but it will never be decreased.
If all
is true
(the default), this function will block repeatedly trying to read all requested bytes, until an error or end-of-file occurs. If all
is false
, at most one read
call is performed, and the amount of data returned is device-dependent. Note that not all stream types support the all
option.
Base.unsafe_read
— Functionunsafe_read(io::IO, ref, nbytes::UInt)
Copy nbytes
from the IO
stream object into ref
(converted to a pointer).
It is recommended that subtypes T<:IO
override the following method signature to provide more efficient implementations: unsafe_read(s::T, p::Ptr{UInt8}, n::UInt)
Base.unsafe_write
— Functionunsafe_write(io::IO, ref, nbytes::UInt)
Copy nbytes
from ref
(converted to a pointer) into the IO
object.
It is recommended that subtypes T<:IO
override the following method signature to provide more efficient implementations: unsafe_write(s::T, p::Ptr{UInt8}, n::UInt)
Base.readeach
— Functionreadeach(io::IO, T)
Return an iterable object yielding read(io, T)
.
See also skipchars
, eachline
, readuntil
.
readeach
requires Julia 1.6 or later.
Examples
julia> io = IOBuffer("JuliaLang is a GitHub organization.\n It has many members.\n");
julia> for c in readeach(io, Char)
c == '\n' && break
print(c)
end
JuliaLang is a GitHub organization.
Base.peek
— Functionpeek(stream[, T=UInt8])
Read and return a value of type T
from a stream without advancing the current position in the stream.
Examples
julia> b = IOBuffer("julia");
julia> peek(b)
0x6a
julia> position(b)
0
julia> peek(b, Char)
'j': ASCII/Unicode U+006A (category Ll: Letter, lowercase)
The method which accepts a type requires Julia 1.5 or later.
Base.position
— Functionposition(s)
Get the current position of a stream.
Examples
julia> io = IOBuffer("JuliaLang is a GitHub organization.");
julia> seek(io, 5);
julia> position(io)
5
julia> skip(io, 10);
julia> position(io)
15
julia> seekend(io);
julia> position(io)
35
Base.seek
— Functionseek(s, pos)
Seek a stream to the given position.
Examples
julia> io = IOBuffer("JuliaLang is a GitHub organization.");
julia> seek(io, 5);
julia> read(io, Char)
'L': ASCII/Unicode U+004C (category Lu: Letter, uppercase)
Base.seekstart
— Functionseekstart(s)
Seek a stream to its beginning.
Examples
julia> io = IOBuffer("JuliaLang is a GitHub organization.");
julia> seek(io, 5);
julia> read(io, Char)
'L': ASCII/Unicode U+004C (category Lu: Letter, uppercase)
julia> seekstart(io);
julia> read(io, Char)
'J': ASCII/Unicode U+004A (category Lu: Letter, uppercase)
Base.seekend
— Functionseekend(s)
Seek a stream to its end.
Base.skip
— Functionskip(s, offset)
Seek a stream relative to the current position.
Examples
julia> io = IOBuffer("JuliaLang is a GitHub organization.");
julia> seek(io, 5);
julia> skip(io, 10);
julia> read(io, Char)
'G': ASCII/Unicode U+0047 (category Lu: Letter, uppercase)
Base.mark
— Functionmark(s)
Add a mark at the current position of stream s
. Return the marked position.
Base.unmark
— Functionunmark(s)
Remove a mark from stream s
. Return true
if the stream was marked, false
otherwise.
Base.reset
— Functionreset(s)
Reset a stream s
to a previously marked position, and remove the mark. Return the previously marked position. Throw an error if the stream is not marked.
Base.ismarked
— FunctionBase.eof
— Functioneof(stream) -> Bool
Test whether an I/O stream is at end-of-file. If the stream is not yet exhausted, this function will block to wait for more data if necessary, and then return false
. Therefore it is always safe to read one byte after seeing eof
return false
. eof
will return false
as long as buffered data is still available, even if the remote end of a connection is closed.
Base.isreadonly
— Functionisreadonly(io) -> Bool
Determine whether a stream is read-only.
Examples
julia> io = IOBuffer("JuliaLang is a GitHub organization");
julia> isreadonly(io)
true
julia> io = IOBuffer();
julia> isreadonly(io)
false
Base.iswritable
— Functioniswritable(io) -> Bool
Return true
if the specified IO object is writable (if that can be determined).
Examples
julia> open("myfile.txt", "w") do io
print(io, "Hello world!");
iswritable(io)
end
true
julia> open("myfile.txt", "r") do io
iswritable(io)
end
false
julia> rm("myfile.txt")
Base.isreadable
— Functionisreadable(io) -> Bool
Return true
if the specified IO object is readable (if that can be determined).
Examples
julia> open("myfile.txt", "w") do io
print(io, "Hello world!");
isreadable(io)
end
false
julia> open("myfile.txt", "r") do io
isreadable(io)
end
true
julia> rm("myfile.txt")
Base.isopen
— Functionisopen(object) -> Bool
Determine whether an object - such as a stream or timer – is not yet closed. Once an object is closed, it will never produce a new event. However, since a closed stream may still have data to read in its buffer, use eof
to check for the ability to read data. Use the FileWatching
package to be notified when a stream might be writable or readable.
Examples
julia> io = open("my_file.txt", "w+");
julia> isopen(io)
true
julia> close(io)
julia> isopen(io)
false
Base.fd
— Functionfd(stream)
Return the file descriptor backing the stream or file. Note that this function only applies to synchronous File
's and IOStream
's not to any of the asynchronous streams.
Base.redirect_stdio
— Functionredirect_stdio(;stdin=stdin, stderr=stderr, stdout=stdout)
Redirect a subset of the streams stdin
, stderr
, stdout
. Each argument must be an IOStream
, TTY
, Pipe
, socket, or devnull
.
redirect_stdio
requires Julia 1.7 or later.
redirect_stdio(f; stdin=nothing, stderr=nothing, stdout=nothing)
Redirect a subset of the streams stdin
, stderr
, stdout
, call f()
and restore each stream.
Possible values for each stream are:
nothing
indicating the stream should not be redirected.path::AbstractString
redirecting the stream to the file atpath
.io
anIOStream
,TTY
,Pipe
, socket, ordevnull
.
Examples
julia> redirect_stdio(stdout="stdout.txt", stderr="stderr.txt") do
print("hello stdout")
print(stderr, "hello stderr")
end
julia> read("stdout.txt", String)
"hello stdout"
julia> read("stderr.txt", String)
"hello stderr"
Edge cases
It is possible to pass the same argument to stdout
and stderr
:
julia> redirect_stdio(stdout="log.txt", stderr="log.txt", stdin=devnull) do
...
end
However it is not supported to pass two distinct descriptors of the same file.
julia> io1 = open("same/path", "w")
julia> io2 = open("same/path", "w")
julia> redirect_stdio(f, stdout=io1, stderr=io2) # not suppored
Also the stdin
argument may not be the same descriptor as stdout
or stderr
.
julia> io = open(...)
julia> redirect_stdio(f, stdout=io, stdin=io) # not supported
redirect_stdio
requires Julia 1.7 or later.
Base.redirect_stdout
— Functionredirect_stdout([stream]) -> stream
Create a pipe to which all C and Julia level stdout
output will be redirected. Return a stream representing the pipe ends. Data written to stdout
may now be read from the rd
end of the pipe.
stream
must be a compatible objects, such as an IOStream
, TTY
, Pipe
, socket, or devnull
.
See also redirect_stdio
.
Base.redirect_stdout
— Methodredirect_stdout(f::Function, stream)
Run the function f
while redirecting stdout
to stream
. Upon completion, stdout
is restored to its prior setting.
Base.redirect_stderr
— Functionredirect_stderr([stream]) -> stream
Like redirect_stdout
, but for stderr
.
stream
must be a compatible objects, such as an IOStream
, TTY
, Pipe
, socket, or devnull
.
See also redirect_stdio
.
Base.redirect_stderr
— Methodredirect_stderr(f::Function, stream)
Run the function f
while redirecting stderr
to stream
. Upon completion, stderr
is restored to its prior setting.
Base.redirect_stdin
— Functionredirect_stdin([stream]) -> stream
Like redirect_stdout
, but for stdin
. Note that the direction of the stream is reversed.
stream
must be a compatible objects, such as an IOStream
, TTY
, Pipe
, socket, or devnull
.
See also redirect_stdio
.
Base.redirect_stdin
— Methodredirect_stdin(f::Function, stream)
Run the function f
while redirecting stdin
to stream
. Upon completion, stdin
is restored to its prior setting.
Base.readchomp
— Functionreadchomp(x)
Read the entirety of x
as a string and remove a single trailing newline if there is one. Equivalent to chomp(read(x, String))
.
Examples
julia> open("my_file.txt", "w") do io
write(io, "JuliaLang is a GitHub organization.\nIt has many members.\n");
end;
julia> readchomp("my_file.txt")
"JuliaLang is a GitHub organization.\nIt has many members."
julia> rm("my_file.txt");
Base.truncate
— Functiontruncate(file, n)
Resize the file or buffer given by the first argument to exactly n
bytes, filling previously unallocated space with '\0' if the file or buffer is grown.
Examples
julia> io = IOBuffer();
julia> write(io, "JuliaLang is a GitHub organization.")
35
julia> truncate(io, 15)
IOBuffer(data=UInt8[...], readable=true, writable=true, seekable=true, append=false, size=15, maxsize=Inf, ptr=16, mark=-1)
julia> String(take!(io))
"JuliaLang is a "
julia> io = IOBuffer();
julia> write(io, "JuliaLang is a GitHub organization.");
julia> truncate(io, 40);
julia> String(take!(io))
"JuliaLang is a GitHub organization.\0\0\0\0\0"
Base.skipchars
— Functionskipchars(predicate, io::IO; linecomment=nothing)
Advance the stream io
such that the next-read character will be the first remaining for which predicate
returns false
. If the keyword argument linecomment
is specified, all characters from that character until the start of the next line are ignored.
Examples
julia> buf = IOBuffer(" text")
IOBuffer(data=UInt8[...], readable=true, writable=false, seekable=true, append=false, size=8, maxsize=Inf, ptr=1, mark=-1)
julia> skipchars(isspace, buf)
IOBuffer(data=UInt8[...], readable=true, writable=false, seekable=true, append=false, size=8, maxsize=Inf, ptr=5, mark=-1)
julia> String(readavailable(buf))
"text"
Base.countlines
— Functioncountlines(io::IO; eol::AbstractChar = '\n')
Read io
until the end of the stream/file and count the number of lines. To specify a file pass the filename as the first argument. EOL markers other than '\n'
are supported by passing them as the second argument. The last non-empty line of io
is counted even if it does not end with the EOL, matching the length returned by eachline
and readlines
.
To count lines of a String
, countlines(IOBuffer(str))
can be used.
Examples
julia> io = IOBuffer("JuliaLang is a GitHub organization.\n");
julia> countlines(io)
1
julia> io = IOBuffer("JuliaLang is a GitHub organization.");
julia> countlines(io)
1
julia> eof(io) # counting lines moves the file pointer
true
julia> io = IOBuffer("JuliaLang is a GitHub organization.");
julia> countlines(io, eol = '.')
1
Base.PipeBuffer
— FunctionPipeBuffer(data::Vector{UInt8}=UInt8[]; maxsize::Integer = typemax(Int))
An IOBuffer
that allows reading and performs writes by appending. Seeking and truncating are not supported. See IOBuffer
for the available constructors. If data
is given, creates a PipeBuffer
to operate on a data vector, optionally specifying a size beyond which the underlying Array
may not be grown.
Base.readavailable
— Functionreadavailable(stream)
Read available buffered data from a stream. Actual I/O is performed only if no data has already been buffered. The result is a Vector{UInt8}
.
The amount of data returned is implementation-dependent; for example it can depend on the internal choice of buffer size. Other functions such as read
should generally be used instead.
Base.IOContext
— TypeIOContext
IOContext
provides a mechanism for passing output configuration settings among show
methods.
In short, it is an immutable dictionary that is a subclass of IO
. It supports standard dictionary operations such as getindex
, and can also be used as an I/O stream.
Base.IOContext
— MethodIOContext(io::IO, KV::Pair...)
Create an IOContext
that wraps a given stream, adding the specified key=>value
pairs to the properties of that stream (note that io
can itself be an IOContext
).
- use
(key => value) in io
to see if this particular combination is in the properties set - use
get(io, key, default)
to retrieve the most recent value for a particular key
The following properties are in common use:
:compact
: Boolean specifying that values should be printed more compactly, e.g. that numbers should be printed with fewer digits. This is set when printing array elements.:compact
output should not contain line breaks.:limit
: Boolean specifying that containers should be truncated, e.g. showing…
in place of most elements.:displaysize
: ATuple{Int,Int}
giving the size in rows and columns to use for text output. This can be used to override the display size for called functions, but to get the size of the screen use thedisplaysize
function.:typeinfo
: aType
characterizing the information already printed concerning the type of the object about to be displayed. This is mainly useful when displaying a collection of objects of the same type, so that redundant type information can be avoided (e.g.[Float16(0)]
can be shown as "Float16[0.0]" instead of "Float16[Float16(0.0)]" : while displaying the elements of the array, the:typeinfo
property will be set toFloat16
).:color
: Boolean specifying whether ANSI color/escape codes are supported/expected. By default, this is determined by whetherio
is a compatible terminal and by any--color
command-line flag whenjulia
was launched.
Examples
julia> io = IOBuffer();
julia> printstyled(IOContext(io, :color => true), "string", color=:red)
julia> String(take!(io))
"\e[31mstring\e[39m"
julia> printstyled(io, "string", color=:red)
julia> String(take!(io))
"string"
julia> print(IOContext(stdout, :compact => false), 1.12341234)
1.12341234
julia> print(IOContext(stdout, :compact => true), 1.12341234)
1.12341
julia> function f(io::IO)
if get(io, :short, false)
print(io, "short")
else
print(io, "loooooong")
end
end
f (generic function with 1 method)
julia> f(stdout)
loooooong
julia> f(IOContext(stdout, :short => true))
short
Base.IOContext
— MethodIOContext(io::IO, context::IOContext)
Create an IOContext
that wraps an alternate IO
but inherits the properties of context
.
Text I/O
Base.show
— Methodshow([io::IO = stdout], x)
Write a text representation of a value x
to the output stream io
. New types T
should overload show(io::IO, x::T)
. The representation used by show
generally includes Julia-specific formatting and type information, and should be parseable Julia code when possible.
repr
returns the output of show
as a string.
To customize human-readable text output for objects of type T
, define show(io::IO, ::MIME"text/plain", ::T)
instead. Checking the :compact
IOContext
property of io
in such methods is recommended, since some containers show their elements by calling this method with :compact => true
.
See also print
, which writes un-decorated representations.
Examples
julia> show("Hello World!")
"Hello World!"
julia> print("Hello World!")
Hello World!
Base.summary
— Functionsummary(io::IO, x)
str = summary(x)
Print to a stream io
, or return a string str
, giving a brief description of a value. By default returns string(typeof(x))
, e.g. Int64
.
For arrays, returns a string of size and type info, e.g. 10-element Array{Int64,1}
.
Examples
julia> summary(1)
"Int64"
julia> summary(zeros(2))
"2-element Vector{Float64}"
Base.print
— Functionprint([io::IO], xs...)
Write to io
(or to the default output stream stdout
if io
is not given) a canonical (un-decorated) text representation. The representation used by print
includes minimal formatting and tries to avoid Julia-specific details.
print
falls back to calling show
, so most types should just define show
. Define print
if your type has a separate "plain" representation. For example, show
displays strings with quotes, and print
displays strings without quotes.
string
returns the output of print
as a string.
Examples
julia> print("Hello World!")
Hello World!
julia> io = IOBuffer();
julia> print(io, "Hello", ' ', :World!)
julia> String(take!(io))
"Hello World!"
Base.println
— Functionprintln([io::IO], xs...)
Print (using print
) xs
followed by a newline. If io
is not supplied, prints to stdout
.
Examples
julia> println("Hello, world")
Hello, world
julia> io = IOBuffer();
julia> println(io, "Hello", ',', " world.")
julia> String(take!(io))
"Hello, world.\n"
Base.printstyled
— Functionprintstyled([io], xs...; bold::Bool=false, underline::Bool=false, blink::Bool=false, reverse::Bool=false, hidden::Bool=false, color::Union{Symbol,Int}=:normal)
Print xs
in a color specified as a symbol or integer, optionally in bold.
color
may take any of the values :normal
, :default
, :bold
, :black
, :blink
, :blue
, :cyan
, :green
, :hidden
, :light_black
, :light_blue
, :light_cyan
, :light_green
, :light_magenta
, :light_red
, :light_yellow
, :magenta
, :nothing
, :red
, :reverse
, :underline
, :white
, or :yellow
or an integer between 0 and 255 inclusive. Note that not all terminals support 256 colors. If the keyword bold
is given as true
, the result will be printed in bold. If the keyword underline
is given as true
, the result will be printed underlined. If the keyword blink
is given as true
, the result will blink. If the keyword reverse
is given as true
, the result will have foreground and background colors inversed. If the keyword hidden
is given as true
, the result will be hidden. Keywords can be given in any combination.
Base.sprint
— Functionsprint(f::Function, args...; context=nothing, sizehint=0)
Call the given function with an I/O stream and the supplied extra arguments. Everything written to this I/O stream is returned as a string. context
can be an IOContext
whose properties will be used, a Pair
specifying a property and its value, or a tuple of Pair
specifying multiple properties and their values. sizehint
suggests the capacity of the buffer (in bytes).
The optional keyword argument context
can be set to a :key=>value
pair, a tuple of :key=>value
pairs, or an IO
or IOContext
object whose attributes are used for the I/O stream passed to f
. The optional sizehint
is a suggested size (in bytes) to allocate for the buffer used to write the string.
Passing a tuple to keyword context
requires Julia 1.7 or later.
Examples
julia> sprint(show, 66.66666; context=:compact => true)
"66.6667"
julia> sprint(showerror, BoundsError([1], 100))
"BoundsError: attempt to access 1-element Vector{Int64} at index [100]"
Base.showerror
— Functionshowerror(io, e)
Show a descriptive representation of an exception object e
. This method is used to display the exception after a call to throw
.
Examples
julia> struct MyException <: Exception
msg::String
end
julia> function Base.showerror(io::IO, err::MyException)
print(io, "MyException: ")
print(io, err.msg)
end
julia> err = MyException("test exception")
MyException("test exception")
julia> sprint(showerror, err)
"MyException: test exception"
julia> throw(MyException("test exception"))
ERROR: MyException: test exception
Base.dump
— Functiondump(x; maxdepth=8)
Show every part of the representation of a value. The depth of the output is truncated at maxdepth
.
Examples
julia> struct MyStruct
x
y
end
julia> x = MyStruct(1, (2,3));
julia> dump(x)
MyStruct
x: Int64 1
y: Tuple{Int64, Int64}
1: Int64 2
2: Int64 3
julia> dump(x; maxdepth = 1)
MyStruct
x: Int64 1
y: Tuple{Int64, Int64}
Base.Meta.@dump
— Macro@dump expr
Show every part of the representation of the given expression. Equivalent to dump(:(expr))
.
Base.readline
— Functionreadline(io::IO=stdin; keep::Bool=false)
readline(filename::AbstractString; keep::Bool=false)
Read a single line of text from the given I/O stream or file (defaults to stdin
). When reading from a file, the text is assumed to be encoded in UTF-8. Lines in the input end with '\n'
or "\r\n"
or the end of an input stream. When keep
is false (as it is by default), these trailing newline characters are removed from the line before it is returned. When keep
is true, they are returned as part of the line.
Examples
julia> open("my_file.txt", "w") do io
write(io, "JuliaLang is a GitHub organization.\nIt has many members.\n");
end
57
julia> readline("my_file.txt")
"JuliaLang is a GitHub organization."
julia> readline("my_file.txt", keep=true)
"JuliaLang is a GitHub organization.\n"
julia> rm("my_file.txt")
Base.readuntil
— Functionreaduntil(stream::IO, delim; keep::Bool = false)
readuntil(filename::AbstractString, delim; keep::Bool = false)
Read a string from an I/O stream or a file, up to the given delimiter. The delimiter can be a UInt8
, AbstractChar
, string, or vector. Keyword argument keep
controls whether the delimiter is included in the result. The text is assumed to be encoded in UTF-8.
Examples
julia> open("my_file.txt", "w") do io
write(io, "JuliaLang is a GitHub organization.\nIt has many members.\n");
end
57
julia> readuntil("my_file.txt", 'L')
"Julia"
julia> readuntil("my_file.txt", '.', keep = true)
"JuliaLang is a GitHub organization."
julia> rm("my_file.txt")
Base.readlines
— Functionreadlines(io::IO=stdin; keep::Bool=false)
readlines(filename::AbstractString; keep::Bool=false)
Read all lines of an I/O stream or a file as a vector of strings. Behavior is equivalent to saving the result of reading readline
repeatedly with the same arguments and saving the resulting lines as a vector of strings.
Examples
julia> open("my_file.txt", "w") do io
write(io, "JuliaLang is a GitHub organization.\nIt has many members.\n");
end
57
julia> readlines("my_file.txt")
2-element Vector{String}:
"JuliaLang is a GitHub organization."
"It has many members."
julia> readlines("my_file.txt", keep=true)
2-element Vector{String}:
"JuliaLang is a GitHub organization.\n"
"It has many members.\n"
julia> rm("my_file.txt")
Base.eachline
— Functioneachline(io::IO=stdin; keep::Bool=false)
eachline(filename::AbstractString; keep::Bool=false)
Create an iterable EachLine
object that will yield each line from an I/O stream or a file. Iteration calls readline
on the stream argument repeatedly with keep
passed through, determining whether trailing end-of-line characters are retained. When called with a file name, the file is opened once at the beginning of iteration and closed at the end. If iteration is interrupted, the file will be closed when the EachLine
object is garbage collected.
To iterate over each line of a String
, eachline(IOBuffer(str))
can be used.
Examples
julia> open("my_file.txt", "w") do io
write(io, "JuliaLang is a GitHub organization.\n It has many members.\n");
end;
julia> for line in eachline("my_file.txt")
print(line)
end
JuliaLang is a GitHub organization. It has many members.
julia> rm("my_file.txt");
Base.displaysize
— Functiondisplaysize([io::IO]) -> (lines, columns)
Return the nominal size of the screen that may be used for rendering output to this IO
object. If no input is provided, the environment variables LINES
and COLUMNS
are read. If those are not set, a default size of (24, 80)
is returned.
Examples
julia> withenv("LINES" => 30, "COLUMNS" => 100) do
displaysize()
end
(30, 100)
To get your TTY size,
julia> displaysize(stdout)
(34, 147)
Multimedia I/O
Just as text output is performed by print
and user-defined types can indicate their textual representation by overloading show
, Julia provides a standardized mechanism for rich multimedia output (such as images, formatted text, or even audio and video), consisting of three parts:
- A function
display(x)
to request the richest available multimedia display of a Julia objectx
(with a plain-text fallback). - Overloading
show
allows one to indicate arbitrary multimedia representations (keyed by standard MIME types) of user-defined types. - Multimedia-capable display backends may be registered by subclassing a generic
AbstractDisplay
type and pushing them onto a stack of display backends viapushdisplay
.
The base Julia runtime provides only plain-text display, but richer displays may be enabled by loading external modules or by using graphical Julia environments (such as the IPython-based IJulia notebook).
Base.Multimedia.AbstractDisplay
— TypeAbstractDisplay
Abstract supertype for rich display output devices. TextDisplay
is a subtype of this.
Base.Multimedia.display
— Functiondisplay(x)
display(d::AbstractDisplay, x)
display(mime, x)
display(d::AbstractDisplay, mime, x)
AbstractDisplay x
using the topmost applicable display in the display stack, typically using the richest supported multimedia output for x
, with plain-text stdout
output as a fallback. The display(d, x)
variant attempts to display x
on the given display d
only, throwing a MethodError
if d
cannot display objects of this type.
In general, you cannot assume that display
output goes to stdout
(unlike print(x)
or show(x)
). For example, display(x)
may open up a separate window with an image. display(x)
means "show x
in the best way you can for the current output device(s)." If you want REPL-like text output that is guaranteed to go to stdout
, use show(stdout, "text/plain", x)
instead.
There are also two variants with a mime
argument (a MIME type string, such as "image/png"
), which attempt to display x
using the requested MIME type only, throwing a MethodError
if this type is not supported by either the display(s) or by x
. With these variants, one can also supply the "raw" data in the requested MIME type by passing x::AbstractString
(for MIME types with text-based storage, such as text/html or application/postscript) or x::Vector{UInt8}
(for binary MIME types).
To customize how instances of a type are displayed, overload show
rather than display
, as explained in the manual section on custom pretty-printing.
Base.Multimedia.redisplay
— Functionredisplay(x)
redisplay(d::AbstractDisplay, x)
redisplay(mime, x)
redisplay(d::AbstractDisplay, mime, x)
By default, the redisplay
functions simply call display
. However, some display backends may override redisplay
to modify an existing display of x
(if any). Using redisplay
is also a hint to the backend that x
may be redisplayed several times, and the backend may choose to defer the display until (for example) the next interactive prompt.
Base.Multimedia.displayable
— Functiondisplayable(mime) -> Bool
displayable(d::AbstractDisplay, mime) -> Bool
Returns a boolean value indicating whether the given mime
type (string) is displayable by any of the displays in the current display stack, or specifically by the display d
in the second variant.
Base.show
— Methodshow(io::IO, mime, x)
The display
functions ultimately call show
in order to write an object x
as a given mime
type to a given I/O stream io
(usually a memory buffer), if possible. In order to provide a rich multimedia representation of a user-defined type T
, it is only necessary to define a new show
method for T
, via: show(io, ::MIME"mime", x::T) = ...
, where mime
is a MIME-type string and the function body calls write
(or similar) to write that representation of x
to io
. (Note that the MIME""
notation only supports literal strings; to construct MIME
types in a more flexible manner use MIME{Symbol("")}
.)
For example, if you define a MyImage
type and know how to write it to a PNG file, you could define a function show(io, ::MIME"image/png", x::MyImage) = ...
to allow your images to be displayed on any PNG-capable AbstractDisplay
(such as IJulia). As usual, be sure to import Base.show
in order to add new methods to the built-in Julia function show
.
Technically, the MIME"mime"
macro defines a singleton type for the given mime
string, which allows us to exploit Julia's dispatch mechanisms in determining how to display objects of any given type.
The default MIME type is MIME"text/plain"
. There is a fallback definition for text/plain
output that calls show
with 2 arguments, so it is not always necessary to add a method for that case. If a type benefits from custom human-readable output though, show(::IO, ::MIME"text/plain", ::T)
should be defined. For example, the Day
type uses 1 day
as the output for the text/plain
MIME type, and Day(1)
as the output of 2-argument show
.
Container types generally implement 3-argument show
by calling show(io, MIME"text/plain"(), x)
for elements x
, with :compact => true
set in an IOContext
passed as the first argument.
Base.Multimedia.showable
— Functionshowable(mime, x)
Returns a boolean value indicating whether or not the object x
can be written as the given mime
type.
(By default, this is determined automatically by the existence of the corresponding show
method for typeof(x)
. Some types provide custom showable
methods; for example, if the available MIME formats depend on the value of x
.)
Examples
julia> showable(MIME("text/plain"), rand(5))
true
julia> showable("image/png", rand(5))
false
Base.repr
— Methodrepr(mime, x; context=nothing)
Returns an AbstractString
or Vector{UInt8}
containing the representation of x
in the requested mime
type, as written by show(io, mime, x)
(throwing a MethodError
if no appropriate show
is available). An AbstractString
is returned for MIME types with textual representations (such as "text/html"
or "application/postscript"
), whereas binary data is returned as Vector{UInt8}
. (The function istextmime(mime)
returns whether or not Julia treats a given mime
type as text.)
The optional keyword argument context
can be set to :key=>value
pair or an IO
or IOContext
object whose attributes are used for the I/O stream passed to show
.
As a special case, if x
is an AbstractString
(for textual MIME types) or a Vector{UInt8}
(for binary MIME types), the repr
function assumes that x
is already in the requested mime
format and simply returns x
. This special case does not apply to the "text/plain"
MIME type. This is useful so that raw data can be passed to display(m::MIME, x)
.
In particular, repr("text/plain", x)
is typically a "pretty-printed" version of x
designed for human consumption. See also repr(x)
to instead return a string corresponding to show(x)
that may be closer to how the value of x
would be entered in Julia.
Examples
julia> A = [1 2; 3 4];
julia> repr("text/plain", A)
"2×2 Matrix{Int64}:\n 1 2\n 3 4"
Base.Multimedia.MIME
— TypeMIME
A type representing a standard internet data format. "MIME" stands for "Multipurpose Internet Mail Extensions", since the standard was originally used to describe multimedia attachments to email messages.
A MIME
object can be passed as the second argument to show
to request output in that format.
Examples
julia> show(stdout, MIME("text/plain"), "hi")
"hi"
Base.Multimedia.@MIME_str
— Macro@MIME_str
A convenience macro for writing MIME
types, typically used when adding methods to show
. For example the syntax show(io::IO, ::MIME"text/html", x::MyType) = ...
could be used to define how to write an HTML representation of MyType
.
As mentioned above, one can also define new display backends. For example, a module that can display PNG images in a window can register this capability with Julia, so that calling display(x)
on types with PNG representations will automatically display the image using the module's window.
In order to define a new display backend, one should first create a subtype D
of the abstract class AbstractDisplay
. Then, for each MIME type (mime
string) that can be displayed on D
, one should define a function display(d::D, ::MIME"mime", x) = ...
that displays x
as that MIME type, usually by calling show(io, mime, x)
or repr(io, mime, x)
. A MethodError
should be thrown if x
cannot be displayed as that MIME type; this is automatic if one calls show
or repr
. Finally, one should define a function display(d::D, x)
that queries showable(mime, x)
for the mime
types supported by D
and displays the "best" one; a MethodError
should be thrown if no supported MIME types are found for x
. Similarly, some subtypes may wish to override redisplay(d::D, ...)
. (Again, one should import Base.display
to add new methods to display
.) The return values of these functions are up to the implementation (since in some cases it may be useful to return a display "handle" of some type). The display functions for D
can then be called directly, but they can also be invoked automatically from display(x)
simply by pushing a new display onto the display-backend stack with:
Base.Multimedia.pushdisplay
— Functionpushdisplay(d::AbstractDisplay)
Pushes a new display d
on top of the global display-backend stack. Calling display(x)
or display(mime, x)
will display x
on the topmost compatible backend in the stack (i.e., the topmost backend that does not throw a MethodError
).
Base.Multimedia.popdisplay
— Functionpopdisplay()
popdisplay(d::AbstractDisplay)
Pop the topmost backend off of the display-backend stack, or the topmost copy of d
in the second variant.
Base.Multimedia.TextDisplay
— TypeTextDisplay(io::IO)
Returns a TextDisplay <: AbstractDisplay
, which displays any object as the text/plain MIME type (by default), writing the text representation to the given I/O stream. (This is how objects are printed in the Julia REPL.)
Base.Multimedia.istextmime
— Functionistextmime(m::MIME)
Determine whether a MIME type is text data. MIME types are assumed to be binary data except for a set of types known to be text data (possibly Unicode).
Examples
julia> istextmime(MIME("text/plain"))
true
julia> istextmime(MIME("image/png"))
false
Network I/O
Base.bytesavailable
— Functionbytesavailable(io)
Return the number of bytes available for reading before a read from this stream or buffer will block.
Examples
julia> io = IOBuffer("JuliaLang is a GitHub organization");
julia> bytesavailable(io)
34
Base.ntoh
— Functionntoh(x)
Convert the endianness of a value from Network byte order (big-endian) to that used by the Host.
Base.hton
— Functionhton(x)
Convert the endianness of a value from that used by the Host to Network byte order (big-endian).
Base.ltoh
— Functionltoh(x)
Convert the endianness of a value from Little-endian to that used by the Host.
Base.htol
— Functionhtol(x)
Convert the endianness of a value from that used by the Host to Little-endian.
Base.ENDIAN_BOM
— ConstantENDIAN_BOM
The 32-bit byte-order-mark indicates the native byte order of the host machine. Little-endian machines will contain the value 0x04030201
. Big-endian machines will contain the value 0x01020304
.