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I would like to share with you how Python uses iterators in recursive functions. I believe most people don't know much about it, so share this article for your reference. I hope you will learn a lot after reading this article. Let's learn about it.

First of all, the function you want to achieve is to recursively traverse the folder, and when you encounter a file that meets the criteria, use yield to return the location of the file.

If you don't need a recursion, you can do this:

Path_list = [] def get_one_cage (root: str, cook_folder_name: str): for item in os.listdir (root). Copy (): item_path = os.path.join (root, item) if item = = cook_folder_name: path_list.append (item_path) return elif os.path.isdir (item_path): get_one_cage (item_path) Cook_folder_name)

That is, depth-first traversal, when the requirement is met, the item_path is added to the list, and then returned to the upper layer.

There is a problem here. You need to have a list that stores all addresses that meet the criteria and takes up memory.

Using an iterator, you can use one to traverse one, saving memory.

Replace it with an iterator, and the first thing that comes to mind is to replace return with yield and call the iterator function using the for loop

Def get_one_cage (root: str, cook_folder_name: str): for item in os.listdir (root). Copy (): item_path = os.path.join (root, item) if item = = cook_folder_name: yield item_path elif os.path.isdir (item_path): get_one_cage (item_path, cook_folder_name)

But when such a program runs to an embedded function, it can't get in, and I'm puzzled.

Now, it should be because the iterator function is not a function, not a command statement, it is just an object.

To put it simply, python programs generally follow the verb + noun structure, or verbs, such as:

A = 1

This sentence actually assigns 1 to a, and there is a verb.

An iterator is just a noun, which must be called with a for statement or a next () method call, or print,yield,return, etc., anyway, you have to add a verb, not a single noun.

And there is a loophole in the above code. In the first piece of code, we use a global variable to hold the traversal results. In the second section of the code, we intended to yield the result to the place where the for loop was called, but the fact is that the program has been wrapped in several layers, and yiled can only return one layer at a time. As shown in the following figure:

To sum up, the two points are amended as follows:

Def get_one_cage (root: str, cook_folder_name: str): for item in os.listdir (root). Copy (): item_path = os.path.join (root, item) if item = = cook_folder_name: yield item_path elif os.path.isdir (item_path): yield get_one_cage (item_path, cook_folder_name)

The execution result of the program is as follows:

Obviously an iterator is returned, not a str, whose logic is shown in the following figure:

It's like, the original intention is:

Xiao Ming passed the sandbag to Xiao Hong, and Xiao Hong passed it to Xiao Lan.

But now it is:

Xiao Ming passed the sandbag to Xiao Hong, and Xiao Hong was passed on.

The modifications are as follows:

Def get_one_cage (root: str, cook_folder_name: str): for item in os.listdir (root). Copy (): item_path = os.path.join (root, item) if item = = cook_folder_name: yield item_path elif os.path.isdir (item_path): yield next (get_one_cage (item_path, cook_folder_name))

The logic is as follows:

Another situation is in the senior source code: use for to call the iterator:

Def get_one_cage (root: str, cook_folder_name: str): for item in os.listdir (root). Copy (): item_path = os.path.join (root, item) if item = = cook_folder_name: yield item_path elif os.path.isdir (item_path): for i in get_one_cage (item_path, cook_folder_name): yield I

This is used for the return of multiple files, and the source code is also used with isfile. Here is a simplified version, so it appears redundant.

Both methods can be used normally.

After writing this article yesterday, I encountered bug. To put it simply, if a folder system does not have the files we want, it will report an error when recursive to the deepest folder.

one

It can be understood as: the boss asks employees to find something, and employees are outsourced to non-staff. If the supernumerary finds what he wants and passes it back all the way, he can hand in the job normally. If it is not found, the supernumerary will keep looking, non-stop, looking for all the places that can be found (traversing the entire folder) and can not find it, it will report an error StopIteration.

Therefore, the crux of the problem is that there is no return mechanism. The way to modify it is to add an empty return at the end of the traversal

Def get_one_cage (root: str): for index, item in enumerate (os.listdir (root)): item_path = os.path.join (root) Item) if item = = 'cooked_xyz': yield item_path elif os.path.isdir (item_path): yield next (get_one_cage (item_path)) elif index = = len (os.listdir (root). Copy ())-1: yield

Or using try... The except statement handles exceptions:

Def get_one_cage (root: str): try: for item in os.listdir (root): item_path = os.path.join (root, item) if item = = 'cooked_xyz': yield item_path elif os.path.isdir (item_path): yield next (get_one_cage (item_path)) except: yield

The error will be reported as above, normal.

It's OK to replace the final yield with return, but it's best to use yield. It's weird to mix the two.

Personally recommend the second method

Note: copy () may not be required.

That's all of the article "how Python uses iterators in recursive functions". Thank you for reading! I believe we all have a certain understanding, hope to share the content to help you, if you want to learn more knowledge, welcome to follow the industry information channel!

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