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Once again, welcome to the How to Python series. In this collection, we explore programming problems that have quick solutions in Python. In this edition, we explore a few ways to check if a file exists in Python, so let’s dive in!
Table of Contents
Recently, I was looking to read some data from a configuration file, but I wanted the code to be backwards compatible. In other words, if the configuration file didn’t exist, I wanted to assume the original preset values. Otherwise, I would pull the data from the configuration file.
Fortunately, I did my research and came up with a solution. The plan was to check to see if the configuration file existed. If it did, the program would read from it and populate the necessary fields. Otherwise, the program would lean back on some arbitrary preset values.
To do that though, I had to find a way to verify the existence of a file. As it turns out, there are plenty of ways to do that in Python.
If we’re looking to check if a file exists, there are a few solutions:
- Check if a file exists with a
try/exceptblock (Python 2+)
- Check if a file exists using
- Check if a file exists using the
Pathobject (Python 3.4+)
Of course, it’s up to us to determine which solution is the best for us!
Check if a File Exists with a Try Block
Up first on the list is a simple try-except block. In this scenario, we would attempt to open our file in the try block. If the file fails to open, we run the preset values. For example:
try: with open('/path/to/file', 'r') as fh: # Store configuration file values except FileNotFoundError: # Keep preset values
This solution is perhaps the simplest and most robust, but the
FileNotFoundError is an update from Python 3. You’ll have more trouble with catching an
IOError in Python 2.
Also, it’s worth noting that we use a
with statement in this example. As an alternative, we can open the file directly. We just have to remember to close it when we’re done.
Check if a File Exists with OS Path
Another option is to skip error handling altogether and directly verify that the path exists. For example:
import os exists = os.path.isfile('/path/to/file') if exists: # Store configuration file values else: # Keep presets
Of course, the drawback here is the race condition from line 2 to line 3. If for some reason the configuration file gets deleted between line 2 and line 3, then the script will crash. If that’s not a risk in your application, then this solution is great.
Check if a File Exists with a Path Object
If you’re obsessed with object-oriented programming like me, then maybe this solution is for you. As of Python 3.4, we can wrap our file reference in an object which brings along a host of new functionality. For example:
from pathlib import Path config = Path('/path/to/file') if config.is_file(): # Store configuration file values else: # Keep presets
In addition, this new object representation allows us to use our original try-except block:
try: absolute_path = config.resolve() # Store configuration file values except FileNotFoundError: # Keep presets
Of course, we may not need all this functionality. After all, if reading the contents is the goal, then the first option is probably the best.
While we’ve already shared all the solutions, it may be important for our application to look at performance as well. To do that, we’ll leverage the
timeit library. First, however, we need to generate a few strings:
setup = """ import os from pathlib import Path """ try_except = """ try: with open('/path/to/file', 'r') as fh: pass except FileNotFoundError: pass """ os_isfile = """ exists = os.path.isfile('/path/to/file') """ path_lib = """ config = Path('/path/to/file') if config.is_file(): pass """
With the strings ready to go, we’ll run this test twice: once where the file exists and again when it doesn’t, respectively.
>>> timeit.timeit(stmt=try_except, setup=setup) 25.758140300000036 >>> timeit.timeit(stmt=os_isfile, setup=setup) 23.302945200000067 >>> timeit.timeit(stmt=path_lib, setup=setup) 36.851380800000015
Normally, we would use the repeat function to try to calculate some sort of lower bound for each function, but it was just way too slow. Feel free to try it and share the results.
For the existing file tests, we’ll have to change the paths in each of the strings above, so they include an existing file. As a result, some of these solutions are significantly slower:
timeit.timeit(stmt=try_except, setup=setup) 220.5547474 >>> timeit.timeit(stmt=os_isfile, setup=setup) 194.13558469999975 >>> timeit.timeit(stmt=path_lib, setup=setup) 208.86859360000017
Here, we can see all of the solutions are quite a bit slower when dealing with an existing file. That said, it seems the
os solution is the fastest in both circumstances. Of course, it does have the race condition drawback, so be sure to take that into account when choosing one of these methods.
For reference, all tests were completed using Windows 10 and Python 3.7.3.
A Little Recap
Using the methods above, we have several options to check if a file exists in Python:
# Brute force with a try-except block try: with open('/path/to/file', 'r') as fh: pass except FileNotFoundError: pass # Leverage the OS package import os exists = os.path.isfile('/path/to/file') # Wrap the path in an object for enhanced functionality from pathlib import Path config = Path('/path/to/file') if config.is_file(): pass
For the purposes of this tutorial, we were only interested in files. However, these solutions can be adapted to verify the existences of directories and symbolic links, so don’t be afraid to play around. That’s the beauty of Python!
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