Python、Pandas、to_csv、os.path.join 参数格式

Question

I am trying to putput a dataframe to csv, and have noticed that the following code works:

df.to_csv(os.path.join(string1, string2 + ".csv"))

However, on the other hand the following code does not work. Could someone kindly help me understand why?

df.to_csv(os.path.join(string1, string2, ".csv"))

Answer 1

The two calls to os.path.join simply return different paths:

>>> import os.path
>>> string1, string2 = "X", "Y"
>>> os.path.join(string1, string2 + ".csv")
'X/Y.csv'
>>> os.path.join(string1, string2, ".csv")
'X/Y/.csv'

The latter will try to save a file called '.csv' in the subdirectory called 'Y' of the directory 'X' . The first code will try to save 'Y.csv' , which is a different file, stored in the 'X' directory.

The two lines give different file paths, so it makes sense that they can produce different behavior, since the in latter the 'Y' directory apparently doesn't exist.

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You can prevent future confusion by using the (somewhat, but not really) new pathlib standard module:

from pathlib import Path

df.to_csv(Path(string1) / f"{string2}.csv")