Print To Standard Error Python
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takes a minute: Sign up How to print to stderr in Python? up vote 675 down vote favorite 103 I've come across at least three ways to print to stderr: import sys print >> sys.stderr, 'spam' sys.stderr.write('spam\n') from __future__ import python redirect stderr print_function print('spam', file=sys.stderr) It seems to contradict zen of Python #13 †, so what's the preferred way to do it? Are there any advantages or disadvantages to one way or the other? † There should be one — and preferably only one — obvious way to do it. python printing stderr zen share|improve this question edited Jun 21 '15 at 22:27 Peter Mortensen 10.3k1369107 asked Apr 7 '11 at 0:59 wim 76.7k24152240 27 The first way listed is one of standard error stream function in python the many things removed in Python 3. The consensus seems to be that the >> syntax was ugly anyway, and since print is now a function, the syntax would never work. –Steve Howard Aug 5 '11 at 21:50 14 Here's another one to add to your list: os.write(2, "spam\n") –Will Hardy Jul 10 '13 at 10:54 4 I use: sys.exit('Error:
concept of standard input, standard output, and standard error. This section is for the rest of you. Standard output and standard error (commonly abbreviated stdout and stderr)
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are pipes that are built into every UNIX system. When you print print(input()) something, it goes to the stdout pipe; when your program crashes and prints out debugging information (like a traceback
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in Python), it goes to the stderr pipe. Both of these pipes are ordinarily just connected to the terminal window where you are working, so when a program prints, you see http://stackoverflow.com/questions/5574702/how-to-print-to-stderr-in-python the output, and when a program crashes, you see the debugging information. (If you're working on a system with a window-based Python IDE, stdout and stderr default to your "Interactive Window".) Example10.8.Introducing stdout and stderr >>> for i in range(3): ... print 'Dive in' Dive in Dive in Dive in >>> import sys >>> for i in range(3): ... sys.stdout.write('Dive in') Dive http://www.diveintopython.net/scripts_and_streams/stdin_stdout_stderr.html inDive inDive in >>> for i in range(3): ... sys.stderr.write('Dive in') Dive inDive inDive in As you saw in Example6.9, "Simple Counters", you can use Python's built-in range function to build simple counter loops that repeat something a set number of times. stdout is a file-like object; calling its write function will print out whatever string you give it. In fact, this is what the print function really does; it adds a carriage return to the end of the string you're printing, and calls sys.stdout.write. In the simplest case, stdout and stderr send their output to the same place: the Python IDE (if you're in one), or the terminal (if you're running Python from the command line). Like stdout, stderr does not add carriage returns for you; if you want them, add them yourself. stdout and stderr are both file-like objects, like the ones you discussed in Section10.1, "Abstracting input sources", but they are both write-only. They have no read method, only write. Still, they are file-like objects, and you can assign any other file- or file-like object to them to redirect their out
2.6. Python 3.0, also known as "Python 3000" or "Py3K", is the first ever intentionally backwards incompatible Python release. https://docs.python.org/3/whatsnew/3.0.html There are more changes than in a typical release, and more https://en.wikibooks.org/wiki/Python_Programming/Input_and_Output that are important for all Python users. Nevertheless, after digesting the changes, you'll find that Python really hasn't changed all that much - by and large, we're mostly fixing well-known annoyances and warts, and removing a lot of old cruft. This article doesn't attempt to in python provide a complete specification of all new features, but instead tries to give a convenient overview. For full details, you should refer to the documentation for Python 3.0, and/or the many PEPs referenced in the text. If you want to understand the complete implementation and design rationale for a particular feature, PEPs usually have more details than print sys.stderr comments the regular documentation; but note that PEPs usually are not kept up-to-date once a feature has been fully implemented. Due to time constraints this document is not as complete as it should have been. As always for a new release, the Misc/NEWS file in the source distribution contains a wealth of detailed information about every small thing that was changed. Common Stumbling Blocks¶ This section lists those few changes that are most likely to trip you up if you're used to Python 2.5. Print Is A Function¶ The print statement has been replaced with a print() function, with keyword arguments to replace most of the special syntax of the old print statement (PEP 3105). Examples: Old: print "The answer is", 2*2 New: print("The answer is", 2*2) Old: print x, # Trailing comma suppresses newline New: print(x, end=" ") # Appends a space instead of a newline Old: print # Prints a newline New: print() # You must call the function! Old: print >>sys.st
1.3.2 Standard File Objects 1.4 Parsing command line 2 Output 2.1 Omitting newlines 2.2 Examples 2.3 File Output 3 External Links Input[edit] Note on Python version: The following uses the syntax of Python 2.x. Some of the following is not going to work with Python 3.x. Python has two functions designed for accepting data directly from the user: input() raw_input() There are also very simple ways of reading a file and, for stricter control over input, reading from stdin if necessary. raw_input()[edit] raw_input() asks the user for a string of data (ended with a newline), and simply returns the string. It can also take an argument, which is displayed as a prompt before the user enters the data. E.g. print raw_input('What is your name? ') prints out What is your name?