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These syntaxes can cause name con icts since they make the imported objects (variables, functions, data types, or modules) directly accessible If we want to use the from import syntax to import lots of objects, we can use multiple lines either by escaping each newline except the last, or by enclosing the object names in parentheses, as the third syntax illustrates In the last syntax, the * means import everything that is not private , which in practical terms means either that every object in the module is imported except for those whose names begin with a leading underscore, or, if the module has a global __all__ variable that holds a list of names, that all the objects named in the __all__ variable are imported Here are a few import examples:
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import os print(ospathbasename(filename)) import ospath as path print(pathbasename(filename)) from os import path print(pathbasename(filename)) # safe fully qualified access
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__all__
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# risk of name collision with path # risk of name collision with path
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from ospath import basename print(basename(filename)) # risk of name collision with basename from ospath import * print(basename(filename)) # risk of many name collisions
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The from importable import * syntax imports all the objects from the module (or all the modules from the package) this could be hundreds of names In the case of from ospath import *, almost 40 names are imported, including dirname, exists, and split, any of which might be names we would prefer to use for our own variables or functions For example, if we write from ospath import dirname, we can conveniently call dirname() without quali cation But if further on in our code we write dirname = "", the object reference dirname will now be bound to the string "" instead of to the dirname() function, so if we try calling dirname() we will get a TypeError exception because dirname now refers to a string and strings are not callable In view of the potential for name collisions the import * syntax creates, some programming teams specify in their guidelines that only the import importable syntax may be used However, certain large packages, particularly GUI (Graphical User Interface) libraries, are often imported this way because they have large numbers of functions and classes (custom data types) that can be tedious to type out by hand A question that naturally arises is, how does Python know where to look for the modules and packages that are imported The built-in sys module has a
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list called syspath that holds a list of the directories that constitute the Python path The rst directory is the directory that contains the program itself, even if the program was invoked from another directory If the PYTHONPATH environment variable is set, the paths speci ed in it are the next ones in the list, and the nal paths are those needed to access Python s standard library these are set when Python is installed When we rst import a module, if it isn t built-in, Python looks for the module in each path listed in syspath in turn One consequence of this is that if we create a module or program with the same name as one of Python s library modules, ours will be found rst, inevitably causing problems To avoid this, never create a program or module with the same name as one of the Python library s top-level directories or modules unless you are providing your own implementation of that module and are deliberately overriding it (A top-level module is one whose py le is in one of the directories in the Python path, rather than in one of those directories subdirectories) For example, on Windows the Python path usually includes a directory called C:\Python31\Lib, so on that platform we should not create a module called Libpy, nor a module with the same name as any of the modules in the C:\Python31\Lib directory One quick way to check whether a module name is in use is to try to import the module This can be done at the console by calling the interpreter with the -c ( execute code ) command-line option followed by an import statement For example, if we want to see whether there is a module called Musicpy (or a top-level directory in the Python path called Music), we can type the following at the console:
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python -c "import Music"
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If we get an ImportError exception we know that no module or top-level directory of that name is in use; any other output (or none) means that the name is taken Unfortunately, this does not guarantee that the name will always be okay, since we might later on install a third-party Python package or module that has a con icting name, although in practice this is a very rare problem For example, if we created a module le called ospy, it would con ict with the library s os module But if we create a module le called pathpy, this would be okay since it would be imported as the path module whereas the library module would be imported as ospath In this book we use an uppercase letter for the rst letter of custom module lenames; this avoids name con icts (at least on Unix) because standard library module lenames are lowercase A program might import some modules which in turn import modules of their own, including some that have already been imported This does not cause any problems Whenever a module is imported Python rst checks to see whether it has already been imported If it has not, Python executes the module s byte-code compiled code, thereby creating the variables, functions, and other objects it provides, and internally records that the module has been imported
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