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This time I'm bringing you python Practical tips ~
Chain comparison operation 
You may think that the process it performs is ：1 < x, return True, Then compare True < 10, Of course, doing so is also a return True, Comparison expression True < 10, Because the interpreter will put True convert to 1,False convert to 0. But the chain comparison interpreter here does not do this internally , It will convert this chain comparison operation into ：1 < x and x < 10, If you don't believe it, you can take a look at the last example . Such chained operation could have been owned by all programming languages , But unfortunately …（https://jq.qq.com/?_wv=1027&k=S5Y5AxaH）
enumeration 
use enumerate Wrap an iteratable object , Iterations and indexes can be used at the same time , If you don't do that , Here is a more troublesome method ：
enumerate You can also receive an optional parameter start, Default start be equal to 0.enumerate(list, start=1), such index The starting value of is 1
what are you having? python I won't answer the related error report 、 Or source code information / Module installation / Women's clothing bosses are proficient in skills You can come here ：（https://jq.qq.com/?_wv=1027&k=2Q3YTfym） Or ask me at the end of the article
Generator object 
You can assign the generator object to x, It means that you can be right x Iterate ：
Its advantage is that there is no need to store intermediate results , Maybe you will use （ List push down ）：（https://jq.qq.com/?_wv=1027&k=S5Y5AxaH）
It's faster than generator objects . relatively , The generator can save more memory , Its value is generated on demand , There is no need to save the whole result in memory like a list push down , At the same time, it cannot iterate again , The list pushdown is not the case .
iter() Acceptable callable Parameters
iter() There are two kinds of parameters received by built-in functions , The first is ：
Parameters collection It must be an iteratable object or a sequence , The second is ：
callable The function will always be called , Until its return result equals sentinel, for example ：
Beware of variable default parameters
Instead, you should use a tag value to indicate “ Is not specified ” To replace variable values , Such as ：（https://jq.qq.com/?_wv=1027&k=S5Y5AxaH）
Send the value to the generator function in
You can ：
If you don't like using space indentation , Then you can use C Language curly braces {} Defined function ：
from __future__ import braces # there braces refer to ：curly braces（ Curly braces ）
File "<stdin>", line 1
SyntaxError: not a chance
Of course, this is just a joke , I want to define a function with curly braces ？ No way . Interested people can also learn about ：
But this is python3 The features inside
Step size parameter in slice operation
There's another exception ：x[::-1], Reverse list ：
About inversion , There are two more functions reverse、reversed,reverse yes list Object method , no return value , and reversed It's a built-in method , Acceptable parameters include tuple、string、list、unicode, And user-defined types , Returns an iterator .
Decorator
Decorators wrap one function or method in another , You can add some additional functions to the wrapped functions , Like logs , You can also set parameters 、 Return the result to modify . The decorator is a little similar Java Medium AOP. The following example is a decorator that prints the parameters in the decorated function ,
@ It's grammar sugar , It is equivalent to ：
for … else grammar 
else The block of code will be in for Execute... After the normal end of the cycle , Unless you meet break( They don't execute ), It is equivalent to the following ：
But this grammar looks strange , It feels like else Block is in for The statement block is executed when it is not executed , It's easy to compare if else The grammar of , If it's a else Switch to finally It may be easier to understand
python2.5 There is one __missing__ Method
dict If a subclass of defines a method __missing__(self, key), If key No longer dict in , that d[key] Will call __missing__ Method , and d[key] The return value of __missing__ The return value of .
stay collections There is a module called defaultdict Of dict Subclass , It is associated with missing Very similar , But for items that don't exist, you don't need to pass parameters .
I hope this article can be helpful to your study now or in the future
Learning to program （python） It is not difficult to
