Another risk is that different Python implementations will do this clean-up at different times. After a file object is closed, either by resume a with statement or by calling ose attempts to use the file object will automatically fail. ose ad Traceback (most recent call last file " stdin line 1, in module valueError: I/O operation on closed file.2.1. Methods of File Objects The rest of the examples in this section will assume that a file object called f has already been created. To read a files contents, call ad(size which reads some quantity of data and returns it as a string (in text mode) or bytes object (in binary mode). Size is an optional numeric argument. When size is omitted or negative, the entire contents of the file will be read and returned; its your problem if the file is twice as large as your machines memory.
This behind-the-scenes modification to file data is fine for text files, but will corrupt binary data like that in jpeg or exe files. Be very careful to use binary mode when reading and writing essay such files. It is good practice to use the with keyword when dealing with file objects. The advantage is that the file is properly closed after its suite finishes, even if an exception is raised at some point. Using with is also much shorter than writing equivalent try - finally blocks: with open workfile. Read_data ad osed True if youre not using the with keyword, then you should call ose to close the file and immediately free up any system resources used. If you dont explicitly close a file, pythons garbage collector will eventually destroy the object and close the open file for you, but the file may stay open for a while.
Mode can be 'r' when the file will only be read, 'w' for only writing (an existing file with the same name will be erased and 'a' opens the file for appending; any data written to the file is automatically added to the end. 'r' opens the file for both reading and writing. The mode argument is optional; 'r' will be assumed if its omitted. Normally, files are opened in text mode, that means, you read and write strings from and to the file, which are encoded in a specific encoding. If encoding is not specified, the default is platform dependent (see open ). 'b' appended to the mode opens the file in binary mode : now the data is read and written in the form of bytes objects. This mode should be used for all files that dont contain text. In text mode, the default when reading is to convert platform-specific line endings (n on Unix, rn on Windows) to just. When writing in text mode, the default is to convert occurrences of n back to platform-specific line endings.
Python, zip file with Example
This can be done by simply passing the dict and using square brackets to access the keys table 'sjoerd 4127, 'jack 4098, 'dcab 8637678 print Jack: 0Jack:d; Sjoerd: 0Sjoerd:d; '. Format(table) Jack: 4098; Sjoerd: 4127; Dcab: 8637678 This could also be done by passing the table as keyword arguments with the * notation. table 'sjoerd 4127, 'jack 4098, 'dcab 8637678 print Jack: Jack:d; Sjoerd: Sjoerd:d; Dcab: Dcab:d'. Format table) Jack: 4098; Sjoerd: 4127; Dcab: 8637678 This is dissertation particularly useful in combination with the built-in function vars, which returns a dictionary containing all local variables. For a complete overview of string story formatting with rmat, see format String Syntax. Old string formatting The operator can also be used for string formatting.
It interprets the left argument much like a sprintf -style format string to be applied to the right argument, and returns the string resulting from this formatting operation. For example: import math print The value of pi is approximately.3f.' math. More information can be found in the printf-style String Formatting section. Reading and Writing Files open returns a file object, and is most commonly used with two arguments: open(filename, mode). f open workfile 'w The first argument is a string containing the filename. The second argument is another string containing a few characters describing the way in which the file will be used.
S' (apply str ) and '! R' (apply repr ) can be used to convert the value before it is formatted: contents 'eels' print my hovercraft is full.'.format(contents) my hovercraft is full of eels. print my hovercraft is full of! Rmat(contents) my hovercraft is full of 'eels'. An optional and format specifier can follow the field name. This allows greater control over how the value is formatted.
The following example rounds pi to three places after the decimal. import math print The value of pi is approximately 0:rmat(math. Pi) The value of pi is approximately.142. Passing an integer after the will cause that field to be a minimum number of characters wide. This is useful for making tables pretty. table 'sjoerd 4127, 'jack 4098, 'dcab 7678 for name, phone in ems. Jack 4098 Dcab 7678 Sjoerd 4127 If you have a really long format string that you dont want to split up, it would be nice if you could reference the variables to be formatted by name instead of by position.
Python, read And, write
' basic usage of the rmat method looks like this: print we are the who say "!.format knights 'ni we are the knights who say "Ni!" The brackets and characters within them (called format fields) are replaced with the objects evernote passed into the rmat method. A number in the brackets can be used to refer to the position of the object passed into the rmat method. print 0 and 1'.format spam 'eggs spam and eggs print 1 and 0'.format spam 'eggs eggs and spam If keyword arguments are used in the rmat method, their values are referred to by using the name of the argument. print This food is rmat(. Food'spam adjective'absolutely horrible this spam is absolutely horrible. Positional and keyword arguments can be arbitrarily combined: print The story of 0, 1, and rmat Bill 'manfred other'georg The story of Bill, manfred, and georg. A' (apply ascii '!
(Note that in the first example, one space between each column was added by the way print works: by default it adds spaces between its arguments.) This example demonstrates the str. Rjust method of essay string objects, which right-justifies a string in a field of a given width by padding it with spaces on the left. There are similar methods str. These methods do not write anything, they just return a new string. If the input string is too long, they dont truncate it, but return it unchanged; this will mess up your column lay-out but thats usually better than the alternative, which would be lying about a value. (If you really want truncation you can always add a slice operation, as. Ljust(n.) There is another method, str. Zfill, which pads a numeric string on the left with zeros. It understands about plus and minus signs: '12'.zfill(5) '00012' '-3.14'.zfill(7) '-003.14' '3.
of x.5, and y is 40000. # The repr of a string adds string"s and backslashes. Hello 'hello, worldn' hellos repr(hello) print(hellos) 'hello, worldn' # The argument to repr may be any python object. Repr(x, y, spam 'eggs.5, 40000, spam 'eggs here are two ways to write a table of squares and cubes: for x in range(1,. # Note use of 'end' on previous line. for x in range(1,. Print 0:2d 1:3d 2:4d'.
The string module contains a, template class which offers yet another way to substitute values into movie strings. One question remains, of course: how do you convert values to strings? Luckily, python has ways to convert any value to a string: pass it to the repr or str functions. The str function is meant to return representations of values which are fairly human-readable, while repr is meant to generate representations which can be read by the interpreter (or will force. SyntaxError if there is no equivalent syntax). For objects which dont have a particular representation for human consumption, str will return the same value as repr. Many values, such as numbers or structures like lists and dictionaries, have the same representation using either function.
File - file handling
There are several ways to present the output of make a program; data can be printed in a human-readable form, or written to a file for future use. This chapter will discuss some of the possibilities. Fancier Output Formatting, so far weve encountered two ways of writing values: expression statements and the print function. (A third way is using the write method of file objects; the standard output file can be referenced as dout. See the library reference for more information on this.). Often youll want more control over the formatting of your output than simply printing space-separated values. There are two ways to format your output; the first way is to do all the string handling yourself; using string slicing and concatenation operations you can create any layout you can imagine. The string type has some methods that perform useful operations for padding strings to a given column width; these will be discussed shortly. The second way is to use formatted string literals, or the rmat method.