Python 2.7 quick reference

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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 not equivalent syntax.

Many values, such as numbers or structures like lists and dictionaries, have the same representation using either function. Strings and floating point numbers, 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 truncates 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 varswhich 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.

However, because this old style of formatting will eventually be removed from the language, str. More information can be found in the Old String Formatting Operations 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 modethat 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. If given an optional parameter sizehintit reads that many bytes from the file and enough more to complete a line, and returns the lines from that. This is often used to allow efficient reading of a large file by lines, but without having to load the entire file in memory.

Only complete lines will be returned. An alternative approach to reading lines is to loop over the file object. This is memory efficient, fast, and leads to simpler code:. The alternative approach is simpler but does not provide as fine-grained control.

Since the two approaches manage line buffering differently, they should not be mixed. In text files those opened without a b in the mode stringonly seeks relative to the beginning of the file are allowed the exception being seeking to the very file end with seek 0, 2. 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 intwhich takes a string like '' and returns its numeric value However, when you want to save more complex data types like lists, dictionaries, or class instances, things get a lot more complicated.

Rather than have users be constantly writing and debugging code to save complicated data types, Python provides a standard module called pickle. This is an amazing module that can take almost any Python object even some forms of Python code! Reconstructing the object from the string representation is called unpickling. Between pickling and unpickling, the string representing the object may have been stored in a file or data, or sent over a network connection to some distant machine.

To unpickle the object again, if f is a file object which has been opened for reading:. Because pickle is so widely used, many authors who write Python extensions take care to ensure that new data types such as matrices can be properly pickled and unpickled. Enter search terms or a module, class or function name. Navigation index modules next previous Python v3. Note use of 'end' on previous line This is the first line of the file. Second line of the file.

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. The pickle Module Previous topic 6.

Modules Next topic 8. The Python Software Foundation is a non-profit corporation. Last updated on Apr 09, Created using Sphinx 0.

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Quite often, we want to embed data values in some explanatory text. For example, if we are displaying the number of nematodes in a hectare, it is a lot more meaningful to display it as "There were So what we need is a way to mix constant text like "nematodes per hectare" with values from elsewhere in your program.

The template is a string containing a mixture of one or more format codes embedded in constant text. The format method uses its arguments to substitute an appropriate value for each format code in the template. The arguments to the. Just to give you the general flavor of how this works, here's a simple conversational example. In the next example, we supply the values using keyword arguments.

The arguments may be supplied in any order. Here is the general form of a format code, where optional parts in [brackets] , and actual characters are in "double quotes":.

The name part of a format code specifies the source of the value to be formatted here. Numbers refer to positional arguments passed to the. You may also use any Python name to refer to one of the keyword arguments. If the associated argument is an iterable, you may append an expression of this form to retrieve one of its elements:. If the associated argument has attributes, you can append an expression of this form to refer to that attribute:.

Starting with Python 2. If you use this convention, you must omit all those numbers. You can, however, omit all the numbers and still use the keyword names feature:. Following the name part of a format code, you can use one of these two forms to force the value to be converted by a standard function:. This character is used to pad a short value to the specified length. It may be specified only in combination with an align character. Specifies how to align values that are not long enough to occupy the specified length.

There are four values:. Here are some examples of the use of fill and align. This option controls whether an arithmetic sign is displayed. There are three possible values:. Place a number at this position to specify the total width of the displayed value. Place a comma at this position in your format code to display commas between groups of three digits in whole numbers. Use this part to specify the number of digits after the decimal point.

This code specifies the general type of format used. The default is to convert the value of a string as if using the str function. Refer to the table below for allowed values. Sometimes you need to format a field using a length that is available only once the program is running. This item then refers to either a positional or keyword argument to the. Suppose you want to format a number n using d digits. Here are examples showing this with and without left-zero fill:.

You can, of course, also use keyword arguments to specify the field width. This trick also works for variable precision. The same technique applies to substituting any of the pieces of a format code. Note This method was added in Python 2. General form of a format code.

If the associated argument is an iterable, you may append an expression of this form to retrieve one of its elements: There are four values: This is the default alignment for string values. This is the default alignment for numbers. Note This feature was added in Python 2. Digits greater than 9 are displayed as lowercase characters. X Display an integer in hexadecimal base Digits greater than 9 are displayed as uppercase characters.

Formatting a field of variable length. The older string format operator. Methods on str values. For numbers using a sign specifier, add the padding between the sign and the rest of the value. Always display a sign: Display - only for negative values. Display one space for positive values, - for negative. Given a number, display the character that has that code. Display a float value using the exponential format. For formatting numbers, this format uses the current local setting to insert separator characters.

Display an integer in hexadecimal base Display a number as a percentage: