PDF saved as raster graphic when plt.savefig is given a Path object #10753

Bug report

I use pathlib quite heavily, and just updated my copy of matplotlib to 2.2.0. I was happy to remove lots of explicit conversion from Path to str in my code, e.g. plt.savefig(str(figure_path)) -> plt.savefig(figure_path).

When passing a pathlib.Path object to plt.savefig, I was surprised to see that the plot was saved as a low-resolution raster image in the PDF, rather than a vector graphic.

Code for reproduction

from pathlib import Path

import matplotlib.pyplot as plt
import numpy as np

x = np.arange(10)
y = np.exp(x)
plt.plot(x, y)

plt.savefig('str_path.pdf')
plt.savefig(Path('pathlib_path.pdf'))

Actual outcome
str_path.pdf
pathlib_path.pdf

Expected outcome

A vector graphic would be saved when using both str paths and pathlib.Path objects.

Matplotlib version

• Operating system: Kubuntu 16.04
• Matplotlib version: 2.2.0, installed via pip, using pre-built binary wheel
• Matplotlib backend (print(matplotlib.get_backend())): TkAgg (though I assume that doesn't apply when saving as PDF)
• Python version: 3.6.4
• Jupyter version (if applicable):
• Other libraries: NumPy 1.14.1 used in reproducible example code