Builtins
input(prompt)Gets user input until the ENTER key is pressed; returns it as a string (without any newline). If there's a prompt string, this is printed to the current prompt line.
Standard input/output
Input readsstdin(usually keyboard), in the same way print writes tostdout(usually the screen).Generally, when we move information between programs, or between programs and hardware, we talk aboutstreams: tubes down which we can send data.Stdin andstdoutcan be regarded as streams from the keyboard to the program, and from the program to the screen.There's also astderrwhere error messages from programs are sent: again, usually the screen by default.
Standard input/output
You can redirect these, for example, at the command prompt:Stdin from file:python a.py < stdin.txtStdoutto overwritten file:python a.py > stdout.txtStdoutto appended file:python a.py >> stdout.txtBoth:python a.py < stdin.txt > stdout.txtYou can alsopipethestdoutof one program to the stdin of another program using the pipe symbol "|" (SHIFT- backslash on most Windows keyboards)
Open
Reading and writing files is a real joy in Python, which makes a complicated job trivial.Thebuiltinopen function is the main method:f = open("filename.txt")for line in f:print(line)f.close()f = open("filename.txt") # Whole file as stringNote the close function (not listed as abuiltin). This is polite - it releases the file.
Open
To write:a = []for i in range(100):a.append("All work and no play makes Jack a dull boy ");f = open("anotherfile.txt", 'w')for line in a:f.write(line)f.close()
Line endings
With "write" you may need to write line endings.The line endings in files vary depending on the operating system.POSIXsystems (Linux; MacOS; etc.) use the ASCII newline character, represented by the escape character\n.Windows uses two characters: ASCII carriage return (\r) (which was used by typewriters to return the typing head to the start of the line), followed by newline.You can find the OS default using theoslibrary:os.linesepBut generally if you use\n, the Python default, Windows copes with it fine, and directly usingos.linesepis advised against.
Seek
It's usual to read an entire file.However, if you want to jump within the file, use:file.seek()https://docs.python.org/3.3/tutorial/inputoutput.html#methods-of-file-objectsorlinecache: Random access to text lineshttps://docs.python.org/3/library/linecache.html
Binary vs text files
The type of the file has a big effect on how we handle it.There are broadly two types of files: text and binary.They are all basically ones and zeros; what is different is how a computer displays them to us, and how much space they take up.
Binary vs. Text files
All files are really just binary 0 and 1 bits.In ‘binary’ files, data is stored in binary representations of the basic types. For example, here's a four byte representations ofintdata:00000000 000000000000000000000000=int000000000 000000000000000000000001 =int100000000 00000000 00000000 00000010 =int200000000 00000000 00000000 00000100 =int400000000 000000000000000000110001 =int4900000000 000000000000000001000001 =int6500000000 00000000 00000000 11111111 =int255
8 bits = 1 byte
Binary vs. Text files
In text files, which can be read in notepad++ etc. characters are often stored in smaller 2-byte areas by code number:00000000 01000001 = code 65 = char “A”00000000 01100001 = code 97 = char “a”
Characters
All chars are part of a set of 16 bit+ international characters called Unicode.These extend the American Standard Code for Information Interchange (ASCII) , which are represented by theints0 to 127, and its superset, the 8 bit ISO-Latin 1 character set (0 to 255).There are some invisible characters used for things like the end of lines.char =chr(8) # Try 7, as well!print("hello" + char + "world")The easiest way to use stuff like newline characters is to useescape characters.print("hello\nworld");
Binary vs. Text files
Note that for an system using 2 byte characters, and 4 byte integers:00000000 00110001 = code 49 = char “1”Seems much smaller – it only uses 2 bytes to store the character “1”, whereas storing theint1 takes 4 bytes.Howevereachcharacter takes this, so:00000000 00110001 = code 49 = char “1”00000000 00110001 00000000 00110010 = code 49, 50 = char “1” “2”00000000 00110001 00000000 0011001000000000 00110111 = code 49, 50, 55 = char “1” “2” “7”Whereas :00000000 000000000000000001111111 =int127
Binary vs. Text files
In short, it is much more efficient to store anything with a lot of numbers as binary (not text).However, as disk space is cheap, networks fast, and it is useful to be able to read data in notepad etc. increasingly people are using text formats like XML.As we’ll see, thefiletypedetermines how we deal with files.
Open
f = open("anotherfile.txt",xxxx)Wherexxxxis (from the docs):
The default mode is 'r' (open for reading text, synonym of 'rt'). For binary read-write access, the mode 'w+b' opens and truncates the file to 0 bytes. 'r+b' opens the file without truncation.
Reading data
f = open("in.txt")data = []for line in f:parsed_line=str.split(line,",")data_line= []for word inparsed_line:data_line.append(float(word))data.append(data_line)print(data)f.close()
Open
Full options:open(file, mode=’r’, buffering=-1, encoding=None, errors=None, newline=None,closefd=True, opener=None)buffering: makes the stream of data more consistent, preventing hardware issues interfering with the process. Generally the default works fine but you can control bytes read in for very large files.encoding: the text file format to use; the default UTF-8 is fine in most cases.errors: how to handle encoding issues, for example the lack of available representations for non-ASCII characters.newline: controls the invisible characters written to mark the end of lines.closefd: whether to remove the file ~link when the file is closed.opener: option for creating more complicated directory and file opening.For more info, see:https://docs.python.org/3/library/functions.html#open
With
The problem with manually closing the file is that exceptions can skip the close statement.Better then, to use the following form:with open("data.txt") as f:for line in f:print(line)Thewithkeyword sets up a Context Manager, which temporarily deals with how the code runs. This closes the file automatically when the clause is left.You can nestwiths, or place more than one on a line, which is equivalent to nesting.with A() as a, B() as b:
Context managers
Context Managers essentially allow pre- or post- execution code to run in the background (likefile.close()).The associated library can also be used to redirectstdout:withcontextlib.redirect_stdout(new_target):For more information, see:https://docs.python.org/3/library/contextlib.html
Reading multiple files
Usefileinputlibrary:importfileinputa = ["file1.txt", "file2.txt", "file3.txt", "file4.txt"]b =fileinput.input(a)for line in b:print(b.filename())print(line)b.close()https://docs.python.org/3/library/fileinput.html
Writing multiple files
importfileinputa = ["file1.txt", "file2.txt", "file3.txt", "file4.txt"]b =fileinput.input(a,inplace=1, backup='.bak')for line in b:print("new text")b.close()'''inplace= 1 # Redirects the stout (i.e. print) to the file.backup ='.bak' # Backs up each file to file1.txt.bak etc. before writing.'''
Easy print to file
print(*objects,sep='', end='\n', file=sys.stdout, flush=False)Prints objects to a file (or stout), separated bysepand followed by end. Other than objects, everything must be akwargas everything else will be written out.Rather than a filename, file must be a proper file object (or anything with a write(string) function).Flushingis the forcible writing of data out of a stream. Occasionally data can be stored in a buffer longer than you might like (for example if another program is reading data as you're writing it, data might get missed is it stays a while in memory), flush forces data writing.
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