Python/ファイルの書き込みと読み込み

ファイルへの書込み

ファイルを書くにも読むにも、まず、そのファイルを開く必要があります。 ファイルを開くには、組込み関数 open() を使います。 もしopen()をするときにその名前のファイルがなければ、ファイルを作成されます。

ファイルへの書込みの例

with open("create.txt", "w") as f:
    print("hello", file=f)

open()の2番目の引数にある「"w"」は、「書込みモード」という意味です。 open()の戻り値は f に保持されます（f の型は _io.TextIOWrapper クラスです）。

open()の関数定義

open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None)

ファイルに書込むには open() の戻り値 f を組込み関数 print() のキーワード引数 file に与えます。 組込み関数 print() は今まで何度も出てきましたが、キーワード引数 file には何も指定していなかったのでディフォルトの sys.stdout （標準出力）が仮定されていました。

オープンしたファイルは必ずクローズする必要があります。 この例では、with文を使っているのでブロックを抜けると自動的にクローズされます。

カレントディレクトリを見ると、「create.txt」という名のファイルが作成されており、開くと「hello」と書かれているはずです。

ファイルからの読込み

ファイルへの一行読込みの例

with open("create.txt") as f:
    s = f.readline()
    print(s)

ここでは組込み関数 open() の第二引数『モード』を与えていませんので、ディフォルトのモード "r"（読込み）になります。 open() の戻り値 f をレシーバーにメソッド readline() を呼び出すと、ファイルから1行ずつ読み込むことができます。

上の例では、ファイルから一行だけ読み込み表示していましたが、全ての行を表示するには

ファイルの全内容を表示する例

with open("create.txt") as f:
    for s in f: 
        print(s, end='')

のように for文 を使います。 print() 関数のキーワード引数 end に を渡しているのは s には改行が含まれているため、 print() のディフォルトの動作「改行を追加する」をキャンセルするためです。

まとめのコード

まとめのコード

with open("/workspace/hello.txt", "w") as f:
    for x in 'city country world universe'.split():
        print(f"hello {x}", file=f)

with open("/workspace/hello.txt") as f:
    s = f.readline()
    print(s)

with open("/workspace/hello.txt") as f:
    for s in f: 
        print(s, end='')
        
with open("/workspace/hello.txt") as f:
    help(f)

実行結果

hello city

hello city
hello country
hello world
hello universe
Help on TextIOWrapper object:

class TextIOWrapper(_TextIOBase)
 |  TextIOWrapper(buffer, encoding=None, errors=None, newline=None, line_buffering=False, write_through=False)
 |  
 |  Character and line based layer over a BufferedIOBase object, buffer.
 |  
 |  encoding gives the name of the encoding that the stream will be
 |  decoded or encoded with. It defaults to locale.getpreferredencoding(False).
 |  
 |  errors determines the strictness of encoding and decoding (see
 |  help(codecs.Codec) or the documentation for codecs.register) and
 |  defaults to "strict".
 |  
 |  newline controls how line endings are handled. It can be None, '',
 |  '\n', '\r', and '\r\n'.  It works as follows:
 |  
 |  * On input, if newline is None, universal newlines mode is
 |    enabled. Lines in the input can end in '\n', '\r', or '\r\n', and
 |    these are translated into '\n' before being returned to the
 |    caller. If it is '', universal newline mode is enabled, but line
 |    endings are returned to the caller untranslated. If it has any of
 |    the other legal values, input lines are only terminated by the given
 |    string, and the line ending is returned to the caller untranslated.
 |  
 |  * On output, if newline is None, any '\n' characters written are
 |    translated to the system default line separator, os.linesep. If
 |    newline is '' or '\n', no translation takes place. If newline is any
 |    of the other legal values, any '\n' characters written are translated
 |    to the given string.
 |  
 |  If line_buffering is True, a call to flush is implied when a call to
 |  write contains a newline character.
 |  
 |  Method resolution order:
 |      TextIOWrapper
 |      _TextIOBase
 |      _IOBase
 |      builtins.object
 |  
 |  Methods defined here:
 |  
 |  __init__(self, /, *args, **kwargs)
 |      Initialize self.  See help(type(self)) for accurate signature.
 |  
 |  __next__(self, /)
 |      Implement next(self).
 |  
 |  __repr__(self, /)
 |      Return repr(self).
 |  
 |  close(self, /)
 |      Flush and close the IO object.
 |      
 |      This method has no effect if the file is already closed.
 |  
 |  detach(self, /)
 |      Separate the underlying buffer from the TextIOBase and return it.
 |      
 |      After the underlying buffer has been detached, the TextIO is in an
 |      unusable state.
 |  
 |  fileno(self, /)
 |      Returns underlying file descriptor if one exists.
 |      
 |      OSError is raised if the IO object does not use a file descriptor.
 |  
 |  flush(self, /)
 |      Flush write buffers, if applicable.
 |      
 |      This is not implemented for read-only and non-blocking streams.
 |  
 |  isatty(self, /)
 |      Return whether this is an 'interactive' stream.
 |      
 |      Return False if it can't be determined.
 |  
 |  read(self, size=-1, /)
 |      Read at most n characters from stream.
 |      
 |      Read from underlying buffer until we have n characters or we hit EOF.
 |      If n is negative or omitted, read until EOF.
 |  
 |  readable(self, /)
 |      Return whether object was opened for reading.
 |      
 |      If False, read() will raise OSError.
 |  
 |  readline(self, size=-1, /)
 |      Read until newline or EOF.
 |      
 |      Returns an empty string if EOF is hit immediately.
 |  
 |  reconfigure(self, /, *, encoding=None, errors=None, newline=None, line_buffering=None, write_through=None)
 |      Reconfigure the text stream with new parameters.
 |      
 |      This also does an implicit stream flush.
 |  
 |  seek(self, cookie, whence=0, /)
 |      Change stream position.
 |      
 |      Change the stream position to the given byte offset. The offset is
 |      interpreted relative to the position indicated by whence.  Values
 |      for whence are:
 |      
 |      * 0 -- start of stream (the default); offset should be zero or positive
 |      * 1 -- current stream position; offset may be negative
 |      * 2 -- end of stream; offset is usually negative
 |      
 |      Return the new absolute position.
 |  
 |  seekable(self, /)
 |      Return whether object supports random access.
 |      
 |      If False, seek(), tell() and truncate() will raise OSError.
 |      This method may need to do a test seek().
 |  
 |  tell(self, /)
 |      Return current stream position.
 |  
 |  truncate(self, pos=None, /)
 |      Truncate file to size bytes.
 |      
 |      File pointer is left unchanged.  Size defaults to the current IO
 |      position as reported by tell().  Returns the new size.
 |  
 |  writable(self, /)
 |      Return whether object was opened for writing.
 |      
 |      If False, write() will raise OSError.
 |  
 |  write(self, text, /)
 |      Write string to stream.
 |      Returns the number of characters written (which is always equal to
 |      the length of the string).
 |  
 |  ----------------------------------------------------------------------
 |  Static methods defined here:
 |  
 |  __new__(*args, **kwargs) from builtins.type
 |      Create and return a new object.  See help(type) for accurate signature.
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors defined here:
 |  
 |  buffer
 |  
 |  closed
 |  
 |  encoding
 |      Encoding of the text stream.
 |      
 |      Subclasses should override.
 |  
 |  errors
 |      The error setting of the decoder or encoder.
 |      
 |      Subclasses should override.
 |  
 |  line_buffering
 |  
 |  name
 |  
 |  newlines
 |      Line endings translated so far.
 |      
 |      Only line endings translated during reading are considered.
 |      
 |      Subclasses should override.
 |  
 |  write_through
 |  
 |  ----------------------------------------------------------------------
 |  Methods inherited from _IOBase:
 |  
 |  __del__(...)
 |  
 |  __enter__(...)
 |  
 |  __exit__(...)
 |  
 |  __iter__(self, /)
 |      Implement iter(self).
 |  
 |  readlines(self, hint=-1, /)
 |      Return a list of lines from the stream.
 |      
 |      hint can be specified to control the number of lines read: no more
 |      lines will be read if the total size (in bytes/characters) of all
 |      lines so far exceeds hint.
 |  
 |  writelines(self, lines, /)
 |      Write a list of lines to stream.
 |      
 |      Line separators are not added, so it is usual for each of the
 |      lines provided to have a line separator at the end.
 |  
 |  ----------------------------------------------------------------------
 |  Data descriptors inherited from _IOBase:
 |  
 |  __dict__