open的原型定義在bultin.py中，是一種內建函數，用於處理文件
open(file, mode='r', buffering=None, encoding=None, errors=None, newline=None, closefd=True)
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Character Meaning
--------- ---------------------------------------------------------------
'r' open for reading (default)只讀
'w' open for writing, truncating the file first 清空內容之后寫內容，並將內容按行截斷
'x' create a new file and open it for writing 如果文件存在就報錯，不存在就創建並寫內容
'a' open for writing, appending to the end of the file if it exists 追加寫內容
'b' binary mode 以字節模式打開，eg mode = 'rb' 以字節打開讀取；如果沒有寫b，則以字符模式讀取
't' text mode (default) 文本模式打開（默認方式）
'+' open a disk file for updating (reading and writing) 為了更新文件，以讀寫方式打開硬盤文件
'U' universal newline mode (deprecated) 全局新行模式（即將棄用）
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def open(file, mode='r', buffering=None, encoding=None, errors=None, newline=None, closefd=True): # known special case of open """ Open file and return a stream. Raise IOError upon failure. file is either a text or byte string giving the name (and the path if the file isn't in the current working directory) of the file to be opened or an integer file descriptor of the file to be wrapped. (If a file descriptor is given, it is closed when the returned I/O object is closed, unless closefd is set to False.) mode is an optional string that specifies the mode in which the file is opened. It defaults to 'r' which means open for reading in text mode. Other common values are 'w' for writing (truncating the file if it already exists), 'x' for creating and writing to a new file, and 'a' for appending (which on some Unix systems, means that all writes append to the end of the file regardless of the current seek position). In text mode, if encoding is not specified the encoding used is platform dependent: locale.getpreferredencoding(False) is called to get the current locale encoding. (For reading and writing raw bytes use binary mode and leave encoding unspecified.) The available modes are: ========= =============================================================== Character Meaning --------- --------------------------------------------------------------- 'r' open for reading (default) 'w' open for writing, truncating the file first 'x' create a new file and open it for writing 'a' open for writing, appending to the end of the file if it exists 'b' binary mode 't' text mode (default) '+' open a disk file for updating (reading and writing) 'U' universal newline mode (deprecated) ========= =============================================================== The default mode is 'rt' (open for reading text). For binary random access, the mode 'w+b' opens and truncates the file to 0 bytes, while 'r+b' opens the file without truncation. The 'x' mode implies 'w' and raises an `FileExistsError` if the file already exists. Python distinguishes between files opened in binary and text modes, even when the underlying operating system doesn't. Files opened in binary mode (appending 'b' to the mode argument) return contents as bytes objects without any decoding. In text mode (the default, or when 't' is appended to the mode argument), the contents of the file are returned as strings, the bytes having been first decoded using a platform-dependent encoding or using the specified encoding if given. 'U' mode is deprecated and will raise an exception in future versions of Python. It has no effect in Python 3. Use newline to control universal newlines mode. buffering is an optional integer used to set the buffering policy. Pass 0 to switch buffering off (only allowed in binary mode), 1 to select line buffering (only usable in text mode), and an integer > 1 to indicate the size of a fixed-size chunk buffer. When no buffering argument is given, the default buffering policy works as follows: * Binary files are buffered in fixed-size chunks; the size of the buffer is chosen using a heuristic trying to determine the underlying device's "block size" and falling back on `io.DEFAULT_BUFFER_SIZE`. On many systems, the buffer will typically be 4096 or 8192 bytes long. * "Interactive" text files (files for which isatty() returns True) use line buffering. Other text files use the policy described above for binary files. encoding is the name of the encoding used to decode or encode the file. This should only be used in text mode. The default encoding is platform dependent, but any encoding supported by Python can be passed. See the codecs module for the list of supported encodings. errors is an optional string that specifies how encoding errors are to be handled---this argument should not be used in binary mode. Pass 'strict' to raise a ValueError exception if there is an encoding error (the default of None has the same effect), or pass 'ignore' to ignore errors. (Note that ignoring encoding errors can lead to data loss.) See the documentation for codecs.register or run 'help(codecs.Codec)' for a list of the permitted encoding error strings. newline controls how universal newlines works (it only applies to text mode). 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 closefd is False, the underlying file descriptor will be kept open when the file is closed. This does not work when a file name is given and must be True in that case. A custom opener can be used by passing a callable as *opener*. The underlying file descriptor for the file object is then obtained by calling *opener* with (*file*, *flags*). *opener* must return an open file descriptor (passing os.open as *opener* results in functionality similar to passing None). open() returns a file object whose type depends on the mode, and through which the standard file operations such as reading and writing are performed. When open() is used to open a file in a text mode ('w', 'r', 'wt', 'rt', etc.), it returns a TextIOWrapper. When used to open a file in a binary mode, the returned class varies: in read binary mode, it returns a BufferedReader; in write binary and append binary modes, it returns a BufferedWriter, and in read/write mode, it returns a BufferedRandom. It is also possible to use a string or bytearray as a file for both reading and writing. For strings StringIO can be used like a file opened in a text mode, and for bytes a BytesIO can be used like a file opened in a binary mode. """ pass
在io.py中，可以看到對open函數的重構，源碼中寫的是io.py中的open函數是builtin.py中open函數的別名(alias)
def open(file, mode='r', buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None): """This is an alias for the builtin open() function. :type file: string :type mode: string :type buffering: numbers.Integral :type encoding: string | None :type errors: string | None :type newline: string | None :type closefd: bool :type opener: ((string, int) -> int) | None :rtype: io.FileIO[bytes] | io.TextIOWrapper[unicode] """ pass
接下來具體看io.py中的幾個常用方法：
f.readline(self, limit=-1) 讀一行，如果提供limit參數，則只能讀一行中的一部分
f.readlines(self, hint=-1) 把文件每一行作為一個list的一個成員，並返回這個list。其實它的內部是通過循環調用readline()來實現的。如果提供hint參數，size是表示讀取內容的總長，也就是說可能只讀到文件的一部分。
def readline(self, limit=-1): """Read and return one line from the stream. :type limit: numbers.Integral :rtype: T """ pass def readlines(self, hint=-1): """Read and return a list of lines from the stream. :type hint: numbers.Integral :rtype: list[T] """ return []
f.write(self, b):
f.writelines(self, lines) 把seq的內容全部寫到文件中(多行一次性寫入)。忠實地寫入，不會在每行后面加上任何東西。
Ref: http://www.cnblogs.com/rollenholt/archive/2012/04/23/2466179.html
未完待續
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1. 打開文件
f = open('db','r+',encoding = 'utf-8') #自動轉換時用utf-8編碼格式將字節轉換成字符串
r+模式，可讀可寫，是最常用的模式
a+模式，可讀可寫，然而由於a是追加寫，不管通過f.seek()將光標移到哪里，最終寫的時候都會把光標移到文件末尾追加
w+模式，可讀可寫，然而在以w+模式打開文件的時候，就清空了原文件了，只是在清空之后是“可讀可寫”的
2. 操作文件
f.read(size) size為讀取的長度，以byte為單位
f.tell() 獲取當前光標的位置
f.seek() 移動光標到指定位置， eg. f.seek(f.tell()) 調整到當前光標所在的位置（字節）
f.write() 從當前光標位置開始向后覆蓋
3. 關閉文件
f.close()
with open(file) as f: #結束時自動關閉文件
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