Python Quick Tutorial (Supplement 02): Python Tips

import module

Import statements are often used in Python to use objects defined in other modules (that is, other .py files).

1) Use __name__

When we write Python library modules, we often run some test statements. When this program is imported as a library, we do not need to run these test statements. One solution is to comment out the test statements in the module before importing. Python has a more elegant solution, which is to use __name__.

The following is a simple library program TestLib.py. When running TestLib.py directly, __name__ is "__main__". If imported, __name__ is "TestLib".

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def lib_func (a):

return a + 10

def lib_func_another(b):

return b + 20

if __name__ == '__main__':

test = 101

PRint(lib_func(test ))

We import the above TestLib in user.py.

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import TestLib

print(TestLib.lib_func(120))

You can try not Use if __name__=='__main__' in TestLib.py and compare the running results.

2) More ways to use import

import TestLib as test # Reference the TestLib module and rename it to t

For example:

For example:

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import TestLib as t

print(t.lib_func(120))

from TestLib import lib_func # Only reference the lib_func object in TestLib and skip TestLib reference field

The advantage of this is to reduce the memory footprint of the referenced module.

For example:

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from Test Lib import lib_func

print(lib_func(120))

from TestLib import *             # Reference all objects in TestLib and skip TestLib reference fields

For example:

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from TestLib import *

print(lib_func(120))

Query

1) Query the parameters of the function

When we want to know which parameters a function will receive, we can use the following method Inquire.

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import inspect

print (inspect.getargspec(func))

2) Query the attributes of an object

In addition to using dir() to query the attributes of an object, we can use the following built-in function to confirm whether an object has a certain attribute:

hasattr(obj, attr_name) # attr_name Is a string

For example:

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a = [1,2,3]

print(hasattr( a,'append'))

2) Query the class and class name to which the object belongs

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a = [1, 2, 3]

print a.__class__

print a.__class__.__name__

3) We can use __base __properties To query the parent class of a certain class:

cls.__base__

For example:

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print(list.__base__)

Use Chinese (and other non-ASCII encodings)

Add #coding=utf8 in the first line of the Python program, for example:

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#coding=utf8

print("How are you?")

can also be used in the following ways:

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#-*- coding: UTF-8 -*-

print("How are you?")

represents binary, octal and hexadecimal numbers

In version 2.6 or above, it is represented in the following way

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print(0b1110) # Binary, starting with 0b

print(0o10) # Octal, starting with 0o

print(0x2A) # Hexadecimal, starting with 0x

If yes For earlier versions, you can use the following method:

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print(int("1110", 2))

print(int("10", 8))

print(int("2A", 16))

Comments

Comments in one line can start with #

Multiple lines Comments can start with "' and end with "', such as

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7. print("Hello world!" ) # use print() function

# main

func()

comments should be aligned with the program block where they are located.

Search path

When we import, Python will look for modules in the search path. For example, the above import TestLib requires TestLib.py to be in the search path.

We can view the search path through the following methods:

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import sys

print(sys.path)

We can add or delete elements in sys.path while Python is running. On the other hand, we can increase the search path for Python by adding the PYTHONPATH environment variable in the shell.

Now we add /home/vamei/mylib to the search path:

$export PYTHONPATH=$PYTHONPATH:/home/vamei/mylib

You can add the front line of command to ~/.bashrc. In this way, we change the search path in the long term.

Combining script and command line

You can use the following method to run a Python script. After the script is finished running, directly enter the Python command line. The advantage of this is that the script object will not be cleared and can be called directly through the command line.

$python -i script.py

Install non-standard packages

Python’s standard library is installed along with Python. When we need non-standard packages, we must install them first.

1) Use linux repository (Linux environment)

This is a good starting point for installing Python add-on packages. You can search for possible Python packages in the Linux repository (for example, search for matplot in the Ubuntu Software Center).

2) Use pip. pip is Python's own package management program. It connects to the Python repository and searches for packages that may exist in it.

For example, use the following method to install, uninstall or upgrade web.py:

$pip install web.py

$pip uninstall web.py

$pip install –upgrade web.py

If your Python is installed in In a non-standard path (use $which python to confirm the path of the python executable file), such as /home/vamei/util/python/bin, you can use the following method to set the path of the pip installation package:

$ pip install –install-option=”–prefix=/home/vamei/util/” web.py

3) Compile from source code

If none of the above methods can find the library you want, you may need to start from the source code Compile. Google is often the best place to start.

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