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Comparing Sequences and Other Types

In document and the Python development team (Page 46-52)

Sequence objects may be compared to other objects with the same sequence type. The comparison uses lex-icographical ordering: first the first two items are compared, and if they differ this determines the outcome of the comparison; if they are equal, the next two items are compared, and so on, until either sequence is exhausted. If two items to be compared are themselves sequences of the same type, the lexicographical com-parison is carried out recursively. If all items of two sequences compare equal, the sequences are considered equal. If one sequence is an initial sub-sequence of the other, the shorter sequence is the smaller (lesser)

Python Tutorial, Release 3.6.4

one. Lexicographical ordering for strings uses the Unicode code point number to order individual characters.

Some examples of comparisons between sequences of the same type:

(1, 2, 3) < (1, 2, 4) [1, 2, 3] < [1, 2, 4]

'ABC' < 'C' < 'Pascal' < 'Python' (1, 2, 3, 4) < (1, 2, 4)

(1, 2) < (1, 2, -1)

(1, 2, 3) == (1.0, 2.0, 3.0)

(1, 2, ('aa', 'ab')) < (1, 2, ('abc', 'a'), 4)

Note that comparing objects of different types with<or>is legal provided that the objects have appropriate comparison methods. For example, mixed numeric types are compared according to their numeric value, so 0 equals 0.0, etc. Otherwise, rather than providing an arbitrary ordering, the interpreter will raise a TypeErrorexception.

5.8. Comparing Sequences and Other Types 41

CHAPTER

SIX

MODULES

If you quit from the Python interpreter and enter it again, the definitions you have made (functions and variables) are lost. Therefore, if you want to write a somewhat longer program, you are better off using a text editor to prepare the input for the interpreter and running it with that file as input instead. This is known as creating ascript. As your program gets longer, you may want to split it into several files for easier maintenance. You may also want to use a handy function that you’ve written in several programs without copying its definition into each program.

To support this, Python has a way to put definitions in a file and use them in a script or in an interactive instance of the interpreter. Such a file is called a module; definitions from a module can be imported into other modules or into themainmodule (the collection of variables that you have access to in a script executed at the top level and in calculator mode).

A module is a file containing Python definitions and statements. The file name is the module name with the suffix .pyappended. Within a module, the module’s name (as a string) is available as the value of the global variable__name__. For instance, use your favorite text editor to create a file called fibo.pyin the current directory with the following contents:

# Fibonacci numbers module

def fib(n): # write Fibonacci series up to n a, b = 0, 1

while b < n:

print(b, end=' ') a, b = b, a+b print()

def fib2(n): # return Fibonacci series up to n result = []

a, b = 0, 1 while b < n:

result.append(b) a, b = b, a+b return result

Now enter the Python interpreter and import this module with the following command:

>>> import fibo

This does not enter the names of the functions defined infibo directly in the current symbol table; it only enters the module namefibothere. Using the module name you can access the functions:

>>> fibo.fib(1000)

1 1 2 3 5 8 13 21 34 55 89 144 233 377 610 987

>>> fibo.fib2(100)

[1, 1, 2, 3, 5, 8, 13, 21, 34, 55, 89]

43

>>> fibo.__name__

'fibo'

If you intend to use a function often you can assign it to a local name:

>>> fib = fibo.fib

>>> fib(500)

1 1 2 3 5 8 13 21 34 55 89 144 233 377

6.1 More on Modules

A module can contain executable statements as well as function definitions. These statements are intended to initialize the module. They are executed only thefirsttime the module name is encountered in an import statement.1 (They are also run if the file is executed as a script.)

Each module has its own private symbol table, which is used as the global symbol table by all functions defined in the module. Thus, the author of a module can use global variables in the module without worrying about accidental clashes with a user’s global variables. On the other hand, if you know what you are doing you can touch a module’s global variables with the same notation used to refer to its functions, modname.itemname.

Modules can import other modules. It is customary but not required to place allimport statements at the beginning of a module (or script, for that matter). The imported module names are placed in the importing module’s global symbol table.

There is a variant of the importstatement that imports names from a module directly into the importing module’s symbol table. For example:

>>> from fibo import fib, fib2

>>> fib(500)

1 1 2 3 5 8 13 21 34 55 89 144 233 377

This does not introduce the module name from which the imports are taken in the local symbol table (so in the example,fibo is not defined).

There is even a variant to import all names that a module defines:

>>> from fibo import *

>>> fib(500)

1 1 2 3 5 8 13 21 34 55 89 144 233 377

This imports all names except those beginning with an underscore (_). In most cases Python programmers do not use this facility since it introduces an unknown set of names into the interpreter, possibly hiding some things you have already defined.

Note that in general the practice of importing* from a module or package is frowned upon, since it often causes poorly readable code. However, it is okay to use it to save typing in interactive sessions.

Note: For efficiency reasons, each module is only imported once per interpreter session. Therefore, if you change your modules, you must restart the interpreter – or, if it’s just one module you want to test interactively, useimportlib.reload(), e.g. import importlib; importlib.reload(modulename).

1 In fact function definitions are also ‘statements’ that are ‘executed’; the execution of a module-level function definition enters the function name in the module’s global symbol table.

Python Tutorial, Release 3.6.4

6.1.1 Executing modules as scripts

When you run a Python module with

python fibo.py <arguments>

the code in the module will be executed, just as if you imported it, but with the__name__set to"__main__".

That means that by adding this code at the end of your module:

if __name__ == "__main__":

import sys

fib(int(sys.argv[1]))

you can make the file usable as a script as well as an importable module, because the code that parses the command line only runs if the module is executed as the “main” file:

$ python fibo.py 50 1 1 2 3 5 8 13 21 34

If the module is imported, the code is not run:

>>> import fibo

>>>

This is often used either to provide a convenient user interface to a module, or for testing purposes (running the module as a script executes a test suite).

6.1.2 The Module Search Path

When a module namedspamis imported, the interpreter first searches for a built-in module with that name.

If not found, it then searches for a file namedspam.pyin a list of directories given by the variablesys.path.

sys.pathis initialized from these locations:

• The directory containing the input script (or the current directory when no file is specified).

• PYTHONPATH (a list of directory names, with the same syntax as the shell variablePATH).

• The installation-dependent default.

Note: On file systems which support symlinks, the directory containing the input script is calculated after the symlink is followed. In other words the directory containing the symlink is not added to the module search path.

After initialization, Python programs can modify sys.path. The directory containing the script being run is placed at the beginning of the search path, ahead of the standard library path. This means that scripts in that directory will be loaded instead of modules of the same name in the library directory. This is an error unless the replacement is intended. See sectionStandard Modulesfor more information.

6.1.3 “Compiled” Python files

To speed up loading modules, Python caches the compiled version of each module in the __pycache__

directory under the namemodule.version.pyc, where the version encodes the format of the compiled file;

it generally contains the Python version number. For example, in CPython release 3.3 the compiled version of spam.py would be cached as__pycache__/spam.cpython-33.pyc. This naming convention allows compiled modules from different releases and different versions of Python to coexist.

6.1. More on Modules 45

Python checks the modification date of the source against the compiled version to see if it’s out of date and needs to be recompiled. This is a completely automatic process. Also, the compiled modules are platform-independent, so the same library can be shared among systems with different architectures.

Python does not check the cache in two circumstances. First, it always recompiles and does not store the result for the module that’s loaded directly from the command line. Second, it does not check the cache if there is no source module. To support a non-source (compiled only) distribution, the compiled module must be in the source directory, and there must not be a source module.

Some tips for experts:

• You can use the-Oor-OOswitches on the Python command to reduce the size of a compiled module.

The-Oswitch removes assert statements, the-OOswitch removes both assert statements and __doc__

strings. Since some programs may rely on having these available, you should only use this option if you know what you’re doing. “Optimized” modules have anopt-tag and are usually smaller. Future releases may change the effects of optimization.

• A program doesn’t run any faster when it is read from a.pycfile than when it is read from a.pyfile;

the only thing that’s faster about.pyc files is the speed with which they are loaded.

• The modulecompileallcan create .pyc files for all modules in a directory.

• There is more detail on this process, including a flow chart of the decisions, in PEP 3147.

In document and the Python development team (Page 46-52)