Performance differences between Python and C

Question

Working on different projects I have the choice of selecting different programming languages, as long as the task is done.

I was wondering what the real difference is, in terms of performance, between writing a program in Python, versus doing it in C.

The tasks to be done are pretty varied, e.g. sorting textfiles, disk access, network access, textfile parsing.

Is there really a noticeable difference between sorting a textfile using the same algorithm in C versus Python, for example?

And in your experience, given the power of current CPU's (i7), is it really a noticeable difference (Consider that its a program that doesnt bring the system to its knees).

Thanks! :)

Answer 1

In general IO bound work will depend more on the algorithm then the language. In this case I would go with Python because it will have first class strings and lots of easy to use libraries for manipulating files, etc.

Answer 2

Is there really a noticeable difference between sorting a textfile using the same algorithm in C versus Python, for example?

Yes.

The noticeable differences are these

1. There's much less Python code.

2. The Python code is much easier to read.

3. Python supports really nice unit testing, so the Python code tends to be higher quality.

4. You can write the Python code more quickly, since there are fewer quirky language features. No preprocessor, for example, really saves a lot of hacking around. Super-experience C programmers hardly notice it. But all that #include sandwich stuff and making the .h files correct is remarkably time-consuming.

5. Python can be easier to package and deploy, since you don't need a big fancy make script to do a build.

Answer 3

Use python until you have a performance problem. If you ever have one figure out what the problem is (often it isn't what you would have guessed up front). Then solve that specific performance problem which will likely be an algorithm or data structure change. In the rare case that your problem really needs C then you can write just that portion in C and use it from your python code.

Answer 4

The first rule of computer performance questions: Your mileage will vary. If small performance differences are important to you, the only way you will get valid information is to test with your configuration, your data, and your benchmark. "Small" here is, say, a factor of two or so.

The second rule of computer performance questions: For most applications, performance doesn't matter -- the easiest way to write the app gives adequate performance, even when the problem scales. If that is the case (and it is usually the case) don't worry about performance.

That said:

• C compiles down to machine executable and thus has the potential to execute as at least as fast as any other language
• Python is generally interpreted and thus may take more CPU than a compiled language
• Very few applications are "CPU bound." I/O (to disk, display, or memory) is not greatly affected by compiled vs interpreted considerations and frequently is a major part of computer time spent on an application
• Python works at a higher level of abstraction than C, so your development and debugging time may be shorter

My advice: Develop in the language you find the easiest with which to work. Get your program working, then check for adequate performance. If, as usual, performance is adequate, you're done. If not, profile your specific app to find out what is taking longer than expected or tolerable. See if and how you can fix that part of the app, and repeat as necessary.

Yes, sometimes you might need to abandon work and start over to get the performance you need. But having a working (albeit slow) version of the app will be a big help in making progress. When you do reach and conquer that performance goal you'll be answering performance questions in SO rather than asking them.

Answer 5

If your text files that you are sorting and parsing are large, use C. If they aren't, it doesn't matter. You can write poor code in any language though. I have seen simple code in C for calculating areas of triangles run 10x slower than other C code, because of poor memory management, use of structures, pointers, etc.

Your I/O algorithm should be independent of your compute algorithm. If this is the case, then using C for the compute algorithm can be much faster.

Answer 6

C will absolutely crush Python in almost any performance category, but C is far more difficult to write and maintain and high performance isn't always worth the trade off of increased time and difficulty in development.

You say you're doing things like text file processing, but what you omit is how much text file processing you're doing. If you're processing 10 million files an hour, you might benefit from writing it in C. But if you're processing 100 files an hour, why not use python? Do you really need to be able to process a text file in 10ms vs 50ms? If you're planning for the future, ask yourself, "Is this something I can just throw more hardware at later?"

Writing solid code in C is hard. Be sure you can justify that investment in effort.

Answer 7

It really depends a lot on what your doing and if the algorithm in question is available in Python via a natively compiled library. If it is, then I believe you'll be looking at performance numbers close enough that Python is most likely your answer -- assuming it's your preferred language. If you must implement the algorithm yourself, depending on the amount of logic required and the size of your data set, C/C++ may be the better option. It's hard to provide a less nebulous answer without more information.

Answer 8

(Assumption - The question implies that the author is familiar with C but not Python, therefore I will base my answer with that in mind.)

I was wondering what the real difference is, in terms of performance, between writing a program in Python, versus doing it in C.

C will almost certainly be faster unless it is implemented poorly, but the real questions are:

• What are the development implications (development time, maintenance, etc.) for either implementation?
• Is the performance benefit significant?

Learning Python can take some time, but there are Python modules that can greatly speed development time. For example, the csv module in Python makes reading and writing csv easy. Also, Python strings, arrays, maps, and other objects make it more flexible than plain C and more elegant, in my opinion, than the equivalent C++. Some things like network access may be much quicker to develop in Python as well.

However, it may take time to learn how to program Python well enough to accomplish your task. Since you are concerned with performance, I suggest trying a simple task, such as sorting a text file, in both C and Python. That will give you a better baseline on both languages in terms of performance, development time, and possibly maintenance.

Answer 9

Across all programs, it isn't really possible to say whether things will be quicker or slower on average in Python or C.

For the programs that I've implemented in both languages, using similar algorithms, I've seen no improvement (and sometimes a performance degradation) for string- and IO-heavy code, when reimplementing python code in C. The execution time is dominated by allocation and manipulation of strings (which functionality python implements very efficiently) and waiting for IO operations (which incurs the same overhead in either language), so the extra overhead of python makes very little difference.

But for programs that do even simple operations on image files, say (images being large enough for processing time to be noticeable compared to IO), C is enormously quicker. For this sort of task the bulk of the time running the python code is spent doing Python Stuff, and this dwarfs the time spent on the underlying operations (multiply, add, compare, etc.). When reimplemented as C, the bureaucracy goes away, the computer spends its time doing real honest work, and for that reason the thing runs much quicker.

It's not uncommon for the python code to run in (say) 5 seconds where the C code runs in (say) 0.05. So that's a 100x increase -- but in absolute terms, this is not so big a deal. It takes so much less longer to write python code than it does to write C code that your program would have to be run some huge number of times to turn a time profit. I often reimplement in C, for various reasons, but if you don't have this requirement then it's probably not worth bothering. You won't get that part of your life back, and next year computers will be quicker.

Answer 10

Actually you can solve most of your tasks efficiently with python.

You just should know which tools to use. For text processing there is brilliant package from Egenix guys - http://www.egenix.com/products/python/mxBase/mxTextTools/. I was able to create very efficient parsers with it in python, since all the heavy lifting is done by native code.

Same approach goes for any other problem - if you have performance problems, get a C/C++ library with Python interface which implements whatever bottleneck you got efficiently.

Answer 11

You will find C is much slower. Your developers will have to keep track of memory allocation, and use libraries (such as glib) to handle simple things such as dictionaries, or lists, which python has built-in.

Moreover, when an error occurs, your C program will typically just crash, which means you'll need to get the error to happen in a debugger. Python would give you a stack trace (typically).

Your code will be bigger, which means it will contain more bugs. So not only will it take longer to write, it will take longer to debug, and will ship with more bugs. This means that customers will notice the bugs more often.

So your developers will spend longer fixing old bugs and thus new features will get done more slowly.

In the mean-time, your competitors will be using a sensible programming language and their products will be increasing in features and usability, rapidly yours will look bad. Your customers will leave and you'll go out of business.

Answer 12

To get an idea of the raw difference in speed, check out the Computer Languages Benchmark Game.

Then you have to decide whether that difference matters to you.

Personally, I ended up deciding that it did, but most of the time instead of using C, I ended up using other higher-level languages. Personally I mostly use Scala, but Haskell and C# and Java each have their advantages also.

Answer 13

The excess time to write the code in C compared to Python will be exponentially greater than the difference between C and Python execution speed.