Should I avoid converting to a string if a value is already a string?

by

Testing if an object is already a string is slower than just always converting to a string.

That’s because the str() method also makes the exact same test (is the object already a string). You are a) doing double the work, and b) your test is slower to boot.

Note: for Python 2, using str() on unicode objects includes an implicit encode to ASCII, and this can fail. You may still have to special case handling of such objects. In Python 3, there is no need to worry about that edge-case.

As there is some discussion around this:

  • isinstance(s, str) has a different meaning when s can be a subclass of str. As subclasses are treated exactly like any other type of object by str() (either __str__ or __repr__ is called on the object), this difference matters here.

  • You should use type(s) is str for exact type checks. Types are singletons, take advantage of this, is is faster:

    >>> import timeit
>>> timeit.timeit("type(s) is str", "s=""")
0.10074466899823165
>>> timeit.timeit("type(s) == str", "s=""")
0.1110201120027341

  • Using s if type(s) is str else str(s) is significantly slower for the non-string case:

    >>> import timeit
>>> timeit.timeit("str(s)", "s = None")
0.1823573520014179
>>> timeit.timeit("s if type(s) is str else str(s)", "s = None")
0.29589492800005246
>>> timeit.timeit("str(s)", "s=""")
0.11716728399915155
>>> timeit.timeit("s if type(s) is str else str(s)", "s=""")
0.12032335300318664

    (The timings for the s="" cases are very close and keep swapping places).

All timings in this post were conducted on Python 3.6.0 on a Macbook Pro 15″ (Mid 2015), OS X 10.12.3.

More Related Contents:

  1. Python string interning
  2. Process escape sequences in a string in Python
  3. Why does substring slicing with index out of range work?
  4. How can I remove the ANSI escape sequences from a string in python
  5. What exactly does the .join() method do?
  6. Python: TypeError: cannot concatenate ‘str’ and ‘int’ objects [duplicate]
  7. Calculate cosine similarity given 2 sentence strings
  8. ‘str’ object does not support item assignment
  9. How can I print variable and string on same line in Python?
  10. Convert a list of characters into a string [duplicate]
  11. inheritance from str or int
  12. Easy way of finding decimal places
  13. How can I select a variable by (string) name?
  14. Splitting a semicolon-separated string to a dictionary, in Python
  15. What is the difference between encode/decode?
  16. How do I write data into CSV format as string (not file)?
  17. How to create a numpy array of arbitrary length strings?
  18. How to write very long string that conforms with PEP8 and prevent E501
  19. Capitalize a string
  20. Right-to-left string replace in Python?
  21. Unknown python expression filename=r’/path/to/file’
  22. Why doesn’t calling a string method (such as .replace) modify (mutate) the string? Why doesn’t it change unless I assign the result?
  23. Formatting long numbers as strings
  24. Adding a string to a list using augmented assignment
  25. How can I interleave or create unique permutations of two strings (without recursion)
  26. How do I convert a list of ascii values to a string in python?
  27. Finding the position of a word in a string
  28. Python split string in moving window
  29. How can I strip first and last double quotes?
  30. Convert bytes to a string in python 3

Leave a Comment