Easily confused concepts in Python-iterators and generators

Iterator and Generator

Iterator (iterator) and generator (generator) are two concepts that are commonly used and easily confused in Python. Today we will sort them out and give some commonly used ones. example.

for statement and iterable object:

for i in [1, 2, 3]:

print(i)

obj = {"a": 123, "b": 456}

for k in obj:

 print(k)

These can The object used in the for statement to loop is an iterable object. In addition to the built-in data types (lists, tuples, strings, dictionaries, etc.) that can be iterated through the for statement, we can also create a container ourselves, containing a series of elements, and each element can be looped out sequentially through the for statement. This A container is an iterator.

The for loop can be used for any sequence type in python, including sequences, tuples and strings. For example:

>>> for x in [1,2,3,4]: print(x * 2,end='')
...
2468

>>> for x in (1,2,3,4): print(x * 2,end='')
...
2468

>>> for y in 'python': print(y * 2 ,end=' ')
...
pp yy tt hh oo nn

Actually, the for loop It's even more general than that: it can be used with any iterable object. You can think of for as an iteration tool, and there are some examples: list parsing, in membership testing, and map built-in functions.

File iterator

File has a method named __next__, which returns the next line in the file each time it is called. It is worth noting that when the end of the file is reached, __next__ will raise the built-in StopIteration exception instead of returning an empty string.

For example:

Note that the print here uses end='' to always add a \n, because the line string already has one (if not At this point, our output will become two lines separated),

There are three advantages to reading files in this way:

1. Simple writing

2. Fast running speed

3. It is also the best in terms of memory usage

The original method with the same effect is to call the readlines method of the file in a for loop , is to load the file into memory and make a list of line strings.

Although the two effects are the same, the latter loads the file into the memory at one time. If the file is too large, the computer memory space is not enough, and it may not even work. The former iterator version has immunity to this problem. (Python3 makes this a bit less obvious by rewriting i/o to support unicode text, and is less dependent on the system)

Of course it can also be implemented using a while loop, but relatively speaking, while is still better than for slow.

Manual iteration: iter and next

In order to support manual iteration of code, python3 also provides a built-in function next, which will automatically use the __next__ method of an object. Given an iterable object z, calling next(z) is equivalent to z.__next__(), but the former is much simpler. For example:

From a technical point of view, when the for loop starts, the iter built-in function will be given through it. Obtains an iterator from an iterable object. The returned object contains the required next method.

Lists, and many other built-in objects, are not iterators themselves because they support opening iterators multiple times. For such an object, we must call iter to start the iteration:

Technically, the for loop calls the internal equivalent of I.__next__ instead of next used here (I)

Now we show the equivalence between automatic and manual iteration:

About try The statement runs an action and captures exceptions that occur during the execution. I will explain in detail in a subsequent article.

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