Python(Head First)學習筆記：四
阿新 • • 發佈：2017-09-22
raise b- before hat contents -- supported between data loss 
4 持久存儲：文件存儲、讀寫
數據保存到文件：在學習的過程中出現了一個問題，老是報一個錯：SyntaxError: invalid syntax；
這個是語法錯誤，後來搜了下才知道是python2.7和python3.5並不兼容，因為之前一直是在ubuntu的終端裏
寫這些簡單的實例，後來程序稍微大點就不方便了，就安裝了idle，用命令：sudo apt-get install idle,安裝完啟動後，
載入python文件，然後運行發現是python2.7,然後逐行運行，發現報錯，而之前這些代碼都是沒問題的，後來重新安
裝idle3,命令：sudo apt-get install idle3，然後啟動：idle3，運行實例代碼，沒有問題。
實例一：
1 Python 3.5.2 (default, Nov 17 2016, 17:05:23) 2 [GCC 5.4.0 20160609] on linux 3 Type "copyright", "credits" or "license()" for more information. 4 >>> import os 5 >>> os.getcwd() 6 ‘/home/user‘ 7 >>> os.chdir(‘/home/user/project/python_model/HeadFirstPython/chapter3‘)View Code8 >>> os.getcwd() 9 ‘/home/user/project/python_model/HeadFirstPython/chapter3‘ 10 >>> man=[] 11 >>> other=[] 12 >>> try: 13 data=open(‘sketch.txt‘) 14 for each_line in data: 15 try: 16 (role,line_spoken)=each_line.split(‘:‘,1) 17 line_spoken=line_spoken.strip()18 if role==‘Man‘: 19 man.append(line_spoken) 20 elif role==‘Other Man‘: 21 other.append(line_spoken) 22 except ValueError: 23 pass 24 data.close() 25 except IOError: 26 print(‘The datafile is missing!‘) 27 28 29 >>> print(man) 30 [‘Is this the right room for an argument?‘, "No you haven‘t!", ‘When?‘, "No you didn‘t!", "You didn‘t!", ‘You did not!‘, ‘Ah! (taking out his wallet and paying) Just the five minutes.‘, ‘You most certainly did not!‘, "Oh no you didn‘t!", "Oh no you didn‘t!", "Oh look, this isn‘t an argument!", "No it isn‘t!", "It‘s just contradiction!", ‘It IS!‘, ‘You just contradicted me!‘, ‘You DID!‘, ‘You did just then!‘, ‘(exasperated) Oh, this is futile!!‘, ‘Yes it is!‘] 31 >>> print(other) 32 ["I‘ve told you once.", ‘Yes I have.‘, ‘Just now.‘, ‘Yes I did!‘, "I‘m telling you, I did!", "Oh I‘m sorry, is this a five minute argument, or the full half hour?", ‘Just the five minutes. Thank you.‘, ‘Anyway, I did.‘, "Now let‘s get one thing quite clear: I most definitely told you!", ‘Oh yes I did!‘, ‘Oh yes I did!‘, ‘Yes it is!‘, "No it isn‘t!", ‘It is NOT!‘, "No I didn‘t!", ‘No no no!‘, ‘Nonsense!‘, "No it isn‘t!"] 33 >>>
以寫模式打開文件
使用open()BIF打開磁盤文件時，可以指定訪問的模式，open()的幫助文件如下：
1 help(open) 2 Help on built-in function open in module io: 3 4 open(file, mode=‘r‘, buffering=-1, encoding=None, errors=None, newline=None, closefd=True, opener=None) 5 Open file and return a stream. Raise IOError upon failure. 6 7 file is either a text or byte string giving the name (and the path 8 if the file isn‘t in the current working directory) of the file to 9 be opened or an integer file descriptor of the file to be 10 wrapped. (If a file descriptor is given, it is closed when the 11 returned I/O object is closed, unless closefd is set to False.) 12 13 mode is an optional string that specifies the mode in which the file 14 is opened. It defaults to ‘r‘ which means open for reading in text 15 mode. Other common values are ‘w‘ for writing (truncating the file if 16 it already exists), ‘x‘ for creating and writing to a new file, and 17 ‘a‘ for appending (which on some Unix systems, means that all writes 18 append to the end of the file regardless of the current seek position). 19 In text mode, if encoding is not specified the encoding used is platform 20 dependent: locale.getpreferredencoding(False) is called to get the 21 current locale encoding. (For reading and writing raw bytes use binary 22 mode and leave encoding unspecified.) The available modes are: 23 24 ========= =============================================================== 25 Character Meaning 26 --------- --------------------------------------------------------------- 27 ‘r‘ open for reading (default) 28 ‘w‘ open for writing, truncating the file first 29 ‘x‘ create a new file and open it for writing 30 ‘a‘ open for writing, appending to the end of the file if it exists 31 ‘b‘ binary mode 32 ‘t‘ text mode (default) 33 ‘+‘ open a disk file for updating (reading and writing) 34 ‘U‘ universal newline mode (deprecated) 35 ========= =============================================================== 36 37 The default mode is ‘rt‘ (open for reading text). For binary random 38 access, the mode ‘w+b‘ opens and truncates the file to 0 bytes, while 39 ‘r+b‘ opens the file without truncation. The ‘x‘ mode implies ‘w‘ and 40 raises an `FileExistsError` if the file already exists. 41 42 Python distinguishes between files opened in binary and text modes, 43 even when the underlying operating system doesn‘t. Files opened in 44 binary mode (appending ‘b‘ to the mode argument) return contents as 45 bytes objects without any decoding. In text mode (the default, or when 46 ‘t‘ is appended to the mode argument), the contents of the file are 47 returned as strings, the bytes having been first decoded using a 48 platform-dependent encoding or using the specified encoding if given. 49 50 ‘U‘ mode is deprecated and will raise an exception in future versions 51 of Python. It has no effect in Python 3. Use newline to control 52 universal newlines mode. 53 54 buffering is an optional integer used to set the buffering policy. 55 Pass 0 to switch buffering off (only allowed in binary mode), 1 to select 56 line buffering (only usable in text mode), and an integer > 1 to indicate 57 the size of a fixed-size chunk buffer. When no buffering argument is 58 given, the default buffering policy works as follows: 59 60 * Binary files are buffered in fixed-size chunks; the size of the buffer 61 is chosen using a heuristic trying to determine the underlying device‘s 62 "block size" and falling back on `io.DEFAULT_BUFFER_SIZE`. 63 On many systems, the buffer will typically be 4096 or 8192 bytes long. 64 65 * "Interactive" text files (files for which isatty() returns True) 66 use line buffering. Other text files use the policy described above 67 for binary files. 68 69 encoding is the name of the encoding used to decode or encode the 70 file. This should only be used in text mode. The default encoding is 71 platform dependent, but any encoding supported by Python can be 72 passed. See the codecs module for the list of supported encodings. 73 74 errors is an optional string that specifies how encoding errors are to 75 be handled---this argument should not be used in binary mode. Pass 76 ‘strict‘ to raise a ValueError exception if there is an encoding error 77 (the default of None has the same effect), or pass ‘ignore‘ to ignore 78 errors. (Note that ignoring encoding errors can lead to data loss.) 79 See the documentation for codecs.register or run ‘help(codecs.Codec)‘ 80 for a list of the permitted encoding error strings. 81 82 newline controls how universal newlines works (it only applies to text 83 mode). It can be None, ‘‘, ‘\n‘, ‘\r‘, and ‘\r\n‘. It works as 84 follows: 85 86 * On input, if newline is None, universal newlines mode is 87 enabled. Lines in the input can end in ‘\n‘, ‘\r‘, or ‘\r\n‘, and 88 these are translated into ‘\n‘ before being returned to the 89 caller. If it is ‘‘, universal newline mode is enabled, but line 90 endings are returned to the caller untranslated. If it has any of 91 the other legal values, input lines are only terminated by the given 92 string, and the line ending is returned to the caller untranslated. 93 94 * On output, if newline is None, any ‘\n‘ characters written are 95 translated to the system default line separator, os.linesep. If 96 newline is ‘‘ or ‘\n‘, no translation takes place. If newline is any 97 of the other legal values, any ‘\n‘ characters written are translated 98 to the given string. 99 100 If closefd is False, the underlying file descriptor will be kept open 101 when the file is closed. This does not work when a file name is given 102 and must be True in that case. 103 104 A custom opener can be used by passing a callable as *opener*. The 105 underlying file descriptor for the file object is then obtained by 106 calling *opener* with (*file*, *flags*). *opener* must return an open 107 file descriptor (passing os.open as *opener* results in functionality 108 similar to passing None). 109 110 open() returns a file object whose type depends on the mode, and 111 through which the standard file operations such as reading and writing 112 are performed. When open() is used to open a file in a text mode (‘w‘, 113 ‘r‘, ‘wt‘, ‘rt‘, etc.), it returns a TextIOWrapper. When used to open 114 a file in a binary mode, the returned class varies: in read binary 115 mode, it returns a BufferedReader; in write binary and append binary 116 modes, it returns a BufferedWriter, and in read/write mode, it returns 117 a BufferedRandom. 118 119 It is also possible to use a string or bytearray as a file for both 120 reading and writing. For strings StringIO can be used like a file 121 opened in a text mode, and for bytes a BytesIO can be used like a file 122 opened in a binary mode.View help
實例二：
1 import os 2 os.getcwd() 3 os.chdir(‘/home/user/project/python_model/HeadFirstPython/chapter3‘) 4 man = [] 5 other = [] 6 try: 7 data = open(‘sketch.txt‘) 8 for each_line in data: 9 try: 10 (role,line_spoken) = each_line.split(‘:‘,1) 11 line_spoken = line_spoken.strip() 12 if role == ‘Man‘: 13 man.append(line_spoken) 14 elif role == ‘Other Man‘: 15 other.append(line_spoken) 16 except ValueError: 17 pass 18 data.close() 19 except IOError: 20 print(‘The datafile is missing!‘) 21 try: 22 man_file = open(‘man_data.txt‘,‘w‘) # open a new file man_data.txt in-mode ‘w‘ 23 other_file = open(‘other_data.txt‘,‘w‘)# if the file don‘t exist then creat it. 24 print(man,file=man_file)# write man data into man_file.txt 25 print(other,file=other_file)# write other data into other_file.txt 26 man_file.close()# close man_file 27 other_file.close()# close other_file 28 except IOError: 29 print(‘File error‘)View Code
註：發生異常後文件會保持打開
為了解決發生異常文件沒有自動關閉的問題，引入finally。
用finally擴展try
在實例二的最後增加：
finally:
man_file.close()
other_file.close()
在python中字符串是不可變的，因為永遠不知道還有哪些變量指向某個特定的字符串；
盡管可以為Python變量賦數值，但實際上變量並不包含所賦的數據；
此外，還有元組也不可以改變，即：不可改變的列表；
所有數值類型也是不可變的。
知道錯誤類型還不夠
如果想知道產生錯誤的具體原因，就需要添加異常處理捕獲機制，如下：
假設現在要打開一個文件：missing.txt，但這個文件並不存在，如下代碼：
try:
data=open(‘missing.txt‘)
print(data.readline(),end=‘‘)
except IOError:
print(‘File error‘)
finally:
if ‘data‘ in locals():
data.close()
繼續改進：
except IOError as err: #為異常對象起一個名
print(‘File error: ‘ + str(err)) #然後作為錯誤消息的一部分
然後運行，結果是：File error:[Errno 2] No such file or directory: ‘missing.txt‘；
但是如果代碼量大了，這種邏輯處理方法會很麻煩，這樣引入with。
用with處理文件
使用以下代碼可以替代上面的try/except/finally代碼：
try:
with open(‘its.txt‘,"w") as data:
print("It‘s...",file=data)
except IOError as err:
print(‘File error:‘ + str(err))
註：使用with時，不需要操心關閉打開文件，Python解釋器會自動處理；
其實，with語句使用了一種名叫：上下文管理協議（context management protocol）的Python技術。
接下來修改第二章筆記中的print_lol()函數
在Python中，標準輸出是：sys.stdout，可以從標準庫sys模塊導入。
實例三
對函數print_lol做修改
def print_lol(the_list,indent=False,level=0,fh=sys.stdout ):
for each_item in the_list:
if isinstance(each_item,list):
print_lol(each_item,indent,level+1,fh)
else:
for tab_stop in range(level):
print("\t" *level,end=‘‘,file=fh)
print(each_item,file=fh)
Python(Head First)學習筆記：四