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There are several ways to present the output of 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.
A third way is using the write method of file objects; the standard output file can be referenced as sys. See the Library Reference for more information on this.
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 the str. The string module contains a Template class which offers yet another way to substitute values into strings.
One question remains, of course: Luckily, Python has ways to convert any value to a string: 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 a SyntaxError if there is no equivalent syntax.
Many values, such as numbers or structures like lists and dictionaries, have the same representation using either function. Strings, in particular, have two distinct representations. Note that in the first example, one space between each column was added by the way print works: This example demonstrates the str. There are similar methods str. These methods do not write anything, they just return a new string.
If you really want truncation you can always add a slice operation, as in x. There is another method, str. It understands about plus and minus signs:. Basic usage of the str. The brackets and characters within them called format fields are replaced with the objects passed into the str. A number in the brackets can be used to refer to the position of the object passed into the str.
If keyword arguments are used in the str. This allows greater control over how the value is formatted. The following example rounds Pi to three places after the decimal. Passing an integer after the ': This is useful for making tables pretty.
This can be done by simply passing the dict and using square brackets '' to access the keys. This is particularly useful in combination with the built-in function vars , which returns a dictionary containing all local variables. For a complete overview of string formatting with str. 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.
More information can be found in the printf-style String Formatting section. 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. 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 the default being UTF Be very careful to use binary mode when reading and writing such files.
The rest of the examples in this section will assume that a file object called f has already been created. Otherwise, at most size bytes are read and returned. If the end of the file has been reached, f. This makes the return value unambiguous; if f. For reading lines from a file, you can loop over the file object.
This is memory efficient, fast, and leads to simple code:. If you want to read all the lines of a file in a list you can also use list f or f. In text files those opened without a b in the mode string , only seeks relative to the beginning of the file are allowed the exception being seeking to the very file end with seek 0, 2 and the only valid offset values are those returned from the f.
Any other offset value produces undefined behaviour. It is good practice to use the with keyword when dealing with file objects. This has the advantage that the file is properly closed after its suite finishes, even if an exception is raised on the way.
It is also much shorter than writing equivalent try - finally blocks:. File objects have some additional methods, such as isatty and truncate which are less frequently used; consult the Library Reference for a complete guide to file objects. Strings can easily be written to and read from a file. Numbers take a bit more effort, since the read method only returns strings, which will have to be passed to a function like int , which takes a string like '' and returns its numeric value When you want to save more complex data types like nested lists and dictionaries, parsing and serializing by hand becomes complicated.
Reconstructing the data from the string representation is called deserializing. Between serializing and deserializing, the string representing the object may have been stored in a file or data, or sent over a network connection to some distant machine. The JSON format is commonly used by modern applications to allow for data exchange. Many programmers are already familiar with it, which makes it a good choice for interoperability.
If you have an object x , you can view its JSON string representation with a simple line of code:. Another variant of the dumps function, called dump , simply serializes the object to a text file. So if f is a text file object opened for writing, we can do this:. To decode the object again, if f is a text file object which has been opened for reading:.
This simple serialization technique can handle lists and dictionaries, but serializing arbitrary class instances in JSON requires a bit of extra effort. The reference for the json module contains an explanation of this. Contrary to JSON , pickle is a protocol which allows the serialization of arbitrarily complex Python objects. As such, it is specific to Python and cannot be used to communicate with applications written in other languages. It is also insecure by default: Enter search terms or a module, class or function name.
Note use of 'end' on previous line This is the first line of the file. Second line of the file. See also pickle - the pickle module Contrary to JSON , pickle is a protocol which allows the serialization of arbitrarily complex Python objects.
Table Of Contents 7. Input and Output 7. Fancier Output Formatting 7. Old string formatting 7. Reading and Writing Files 7. Methods of File Objects 7. Saving structured data with json Previous topic 6. Modules Next topic 8. The Python Software Foundation is a non-profit corporation. Last updated on Sep 19, Created using Sphinx 1.