2020--Python grammar common knowledge points

“A quitter never wins and a winner never quits” — Napoleon Hill

3.1 Garbage Collection##

• [ In-depth understanding of Python memory management and garbage collection, no longer afraid to ask (1)](https://links.jianshu.com/go?to=https%3A%2F%2Fyuzhoustayhungry.github.io%2Fpost%2Fpython%25E5 %2586%2585%25E5%25AD%2598%25E7%25AE%25A1%25E7%2590%2586%25E4%25B8%258E%25E5%259E%2583%25E5%259C%25BE%25E5%259B%259E%25E6%2594 %25B6%2F)
• [ Also talk about Python's garbage collection mechanism? ](https://links.jianshu.com/go?to=https%3A%2F%2Fyuzhoustayhungry.github.io%2Fpost%2Fpython%25E5%259E%2583%25E5%259C%25BE%25E5%259B%259E% 25E6%2594%25B6%2F)

Garbage collection is the process of finding data objects that will not be accessible in the future in a running program, and reclaiming the resources (especially memory) used by those objects. Languages for automatic garbage collection--Java, C#, Python and most scripting languages. C is a language without garbage collection-programmers need to know when to allocate and reclaim memory.

• Most garbage collection languages use reference counting (reference counting, tracking the number of references to an object) or reachability analysis (tracing, starting from the "root" object, looking for objects that can be accessed by a series of references, and the rest Objects that are not referenced will be treated as "garbage" and collected).
• Python uses reference counting. The advantage is that when the reference count becomes 0, it can immediately reclaim the object. The cost of this is that an additional integer value is required for each [Object Storage] (https://cloud.tencent.com/product/cos?from=10680). The benefit of reachability analysis (used in Java) is that it can be executed in a separate thread, thereby improving performance. The disadvantage is that when the garbage collector is running, the program will suspend all threads.
• The disadvantage of reference counting is that it cannot solve "circular references". For example, object A and object B refer to each other, Ax = B and By = A, so the reference count will not change to below 1, even if there are no other objects Reference A and B, in this case, the garbage collector will periodically look for and delete them.
• The garbage collector uses heuristics to increase speed. For example, based on experience, recently created objects are more likely to die. Therefore, when creating objects, assign them to different generations, and then check the young generation first.

3.2 Deep copy and shallow copy##

Refer to the previous article: [Learning Python for a year, this time I finally understand shallow copy and deep copy] (https://links.jianshu.com/go?to=https%3A%2F%2Fyuzhoustayhungry.github.io%2Fpost%2F%25E6%25B7%25B1%25E6%258B%25B7%25E8%25B4%259D%25E5%2592%258C%25E6%25B5%2585%25E6%258B%25B7%25E8%25B4%259D%2F)

Q: What is the difference between shallow copy and deep copy?

answer:

• copy.copy(x) and copy.deepcopy(x), shallow copy will construct a new composite object, and then (to the extent possible) insert references to the objects found in the original object. Deep copy will construct a new compound object, and then recursively insert a copy of the object in the original object.
• The difference between shallow copy and deep copy is only related to compound objects, which are objects that contain other objects (such as lists or class instances).
• If you know that the client will not change the object, you can use copy less. Similarly, if the object itself is immutable, such as a tuple, there is no need to copy it.

3.3 Iterators and generators##

Q: What is the difference between an iterator and a generator?

**Iterator: ** An object that implements the __iter__() and __next__() methods. The first method returns the iterator object itself and is used in for and in statements. The first method returns the next value in the iteration. If there are no more elements, a StopIteration exception will be raised.

**Generator: ** An easy way to create iterators, use the keyword yield. Generators use the function call stack to implicitly store the state of the iterator-compared to writing the same iterator as an explicit class, it simplifies the writing of iterators. It also helps improve readability.

Every generator is an iterator, but the reverse is incorrect. In particular, iterators can be fully mature classes and can therefore provide other functions. For example, it is easy to add a method to the iterator class above to change the iteration limit, which is impossible for generators.

3.4 Decorator@decorator

@ Symbols are syntactic sugar for decorators. They are used when defining functions to avoid assignment operations again.

import time

def time_function(f):
 def wrapper(*args,**kwargs):
  begin = time.time()
  result =f(*args,**kwargs)
  end = time.time()print("Function call with argument {all_args} took ".format(
   all_args="\t".join((str(args),str(kwargs))))+str(end - begin)+" seconds to execute.")return result

 return wrapper

3.5 List vs. tuple

Lists[1, 4, 7, "apple", 4], Tuple(3.14, "PI", 2,43, "e")

**The same point: ** are containers, and they can store different types of data, and can be indexed for access a[i]

**Difference: ** The tuple is immutable, you cannot change the value of the index a[i], nor can you add/remove elements from the tuple; but the list can.

Benefits of immutability: performance improvement, container friendliness, thread safety. The ancestor can be placed in the set set and used as a key value, but a list cannot. The creation and access speed of tuples is slightly faster, and the memory footprint is small.

3.6 * args and *kwargs

Both are used to pass variable parameters in functions. *arg is used to pass variable-length parameter lists:

• It does not matter that the parameter referenced in the function is called args-it can also be called A or varargs, args is an idiom;
• * Must be followed by regular parameters

The second parameter **kwargs is used when passing a variable number of keyword arguments to the function.

• It is not important to call the parameters referenced in the function args, you can also call them D or argdict.
• ** Parameters must appear after all regular named parameters and * parameters
• Keyword actual participation in named actual parameters is very different, the name of the named actual parameter is specified in the function itself

3.7 Error handling, exception mechanism##

3.8 lambda

increment_by_i =[lambda x: x + i for i inrange(10)]print(increment_by_i[3](4))

The program will print 13 (= 9 + 4) instead of the expected 7 (= 3 + 4). This is because the functions created in the loop have the same scope. They use the same variable name, so they all refer to the same variable i, which is 10 at the end of the loop, so it is 13 (= 9 + 4).

There are many ways to get the desired behavior. The reasonable way is to return the lambda from the function to avoid naming conflicts.

3.9 Function parameters##

What are the positional parameters, keyword parameters and default parameters of a function.