python - What does -1 mean in this code([file for file in os.listdir(folder_dir) if file.find("xlsx") != -1])?

Question

How can you interpret the code?

[file for file in os.listdir(folder_dir) if file.find("xlsx") != -1]

Also, what does -1 mean in this code?

question from:https://stackoverflow.com/questions/65662235/what-does-1-mean-in-this-codefile-for-file-in-os-listdirfolder-dir-if-file

Answer 1

The string find function gives -1 automatically if it does not appear in the string. So your code is doing a condition (if statement) to check whether xlsx is in the string.

The str.find function is meant to find the index of the substring being in the string.

Here is an example:

>>> a = '123abc'
>>> a.find('a')
3
>>> a.find('Something that is not in the string')
-1
>>> 

As mentioned in the documentation:

Return the lowest index in the string where substring sub is found within the slice s[start:end]. Optional arguments start and end are interpreted as in slice notation. Return -1 if sub is not found.