There is a trend of discouraging setting sys.setdefaultencoding('utf-8') in Python 2. Can anybody list real examples of problems with that? Arguments like it is harmful or it hides bugs don't sound very convincing.
UPDATE: Please note that this question is only about utf-8, it is not about changing default encoding "in general case".
Please give some examples with code if you can.
The original poster asked for code which demonstrates that the switch is harmful—except that it "hides" bugs unrelated to the switch.
Updates
[2020-11-01]: pip install setdefaultencoding
Eradicates the need to reload(sys) (from Thomas
Grainger).
[2019]: Personal experience with python3:
No unicode en/decoding problems. Reasons:
Got used to writing .encode('utf-8') .decode('utf-8') a (felt) 100 times a day.
Looking into libraries: Same. 'utf-8' either hardcoded or the silent default, in pretty much all the I/O done
Heavily improved byte strings support made it finally possible to convert I/O centric applications like mercurial.
Having to write .encode and .decode all the time got people aware of the difference between strings for humans and machines.
In my opinion, python2's bytestrings combined with (utf-8 default) decoding only before outputting to humans or unicode only formats would have been the technical superior approach, compared to decoding/encoding everything at ingress and at egress w/o actual need many many times. It depends on the application if something like the len() function is more practical, when returning the character count for humans, compared to returning the bytes used to store and forward by machines.
=> I think it's safe to say that UTF-8 everywhere saved the Unicode Sandwich Design.
Without that many libraries and applications, which only pass through strings w/o interpreting them could not work.
Summary of conclusions
(from 2017)
Based on both experience and evidence I've collected, here are the conclusions I've arrived at.
Setting the defaultencoding to UTF-8 nowadays is safe, except for specialised applications, handling files from non unicode ready systems.
The "official" rejection of the switch is based on reasons no longer relevant for a vast majority of end users (not library providers), so we should stop discouraging users to set it.
Working in a model that handles Unicode properly by default is far better suited for applications for inter-systems communications than manually working with unicode APIs.
Effectively, modifying the default encoding very frequently avoids a number of user headaches in the vast majority of use cases. Yes, there are situations in which programs dealing with multiple encodings will silently misbehave, but since this switch can be enabled piecemeal, this is not a problem in end-user code.
More importantly, enabling this flag is a real advantage is users' code, both by reducing the overhead of having to manually handle Unicode conversions, cluttering the code and making it less readable, but also by avoiding potential bugs when the programmer fails to do this properly in all cases.
Since these claims are pretty much the exact opposite of Python's official line of communication, I think the an explanation for these conclusions is warranted.
Examples of successfully using a modified defaultencoding in the wild
Dave Malcom of Fedora believed it is always right. He proposed, after investigating risks, to change distribution wide def.enc.=UTF-8 for all Fedora users.
Hard fact presented though why Python would break is only the hashing behavior I listed, which is never picked up by any other opponent within the core community as a reason to worry about or even by the same person, when working on user tickets.
Resume of Fedora: Admittedly, the change itself was described as "wildly unpopular" with the core developers, and it was accused of being inconsistent with previous versions.
There are 3000 projects alone at openhub doing it. They have a slow search frontend, but scanning over it, I estimate 98% are using UTF-8. Nothing found about nasty surprises.
There are 18000(!) github master branches with it changed.
While the change is "unpopular" at the core community its pretty popular in the user base. Though this could be disregarded, since users are known to use hacky solutions, I don't think this is a relevant argument due to my next point.
There are only 150 bugreports total on GitHub due to this. At a rate of effectively 100%, the change seems to be positive, not negative.
To summarize the existing issues people have run into, I've scanned through all of the aforementioned tickets.
Chaging def.enc. to UTF-8 is typically introduced but not removed in the issue closing process, most often as a solution. Some bigger ones excusing it as temporary fix, considering the "bad press" it has, but far more bug reporters are justglad about the fix.
A few (1-5?) projects modified their code doing the type conversions manually so that they did not need to change the default anymore.
In two instances I see someone claiming that with def.enc. set to UTF-8 leads to a complete lack of output entirely, without explaining the test setup. I could not verify the claim, and I tested one and found the opposite to be true.
One claims his "system" might depend on not changing it but we do not learn why.
One (and only one) had a real reason to avoid it: ipython either uses a 3rd party module or the test runner modified their process in an uncontrolled way (it is never disputed that a def.enc. change is advocated by its proponents only at interpreter setup time, i.e. when 'owning' the process).
I found zero indication that the different hashes of 'é' and u'é' causes problems in real-world code.
Python does not "break"
After changing the setting to UTF-8, no feature of Python covered by unit tests is working any differently than without the switch. The switch itself, though, is not tested at all.
It is advised on bugs.python.org to frustrated users
Examples here, here or here
(often connected with the official line of warning)
The first one demonstrates how established the switch is in Asia (compare also with the github argument).
Ian Bicking published his support for always enabling this behavior.
I can make my systems and communications consistently UTF-8, things will just get better. I really don't see a downside. But why does Python make it SO DAMN HARD [...] I feel like someone decided they were smarter than me, but I'm not sure I believe them.
Martijn Fassen, while refuting Ian, admitted that ASCII might have been wrong in the first place.
I believe if, say, Python 2.5, shipped with a default encoding of UTF-8, it wouldn't actually break anything. But if I did it for my Python, I'd have problems soon as I gave my code to someone else.
In Python3, they don't "practice what they preach"
While opposing any def.enc. change so harshly because of environment dependent code or implicitness, a discussion here revolves about Python3's problems with its 'unicode sandwich' paradigm and the corresponding required implicit assumptions.
Further they created possibilities to write valid Python3 code like:
>>> from 褐褑褒褓褔褕褖褗褘 import *
>>> def 空手(合氣道): あいき(ど(合氣道))
>>> 空手(う힑힜('👏 ') + 흾)
💔
DiveIntoPython recommends it.
In this thread, Guido himself advises a professional end user to use a process specific environt with the switch set to "create a custom Python environment for each project."
The fundamental reason the designers of Python's 2.x standard library don't want you to be able to set the default encoding in your app, is that the standard library is written with the assumption that the default encoding is fixed, and no guarantees about the correct workings of the standard library can be made when you change it. There are no tests for this situation. Nobody knows what will fail when. And you (or worse, your users) will come back to us with complaints if the standard library suddenly starts doing things you didn't expect.
Jython offers to change it on the fly, even in modules.
PyPy did not support reload(sys) - but brought it back on user request within a single day without questions asked. Compare with the "you are doing it wrong" attitude of CPython, claiming without proof it is the "root of evil".
Ending this list I confirm that one could construct a module which crashes because of a changed interpreter config, doing something like this:
def is_clean_ascii(s):
""" [Stupid] type agnostic checker if only ASCII chars are contained in s"""
try:
unicode(str(s))
# we end here also for NON ascii if the def.enc. was changed
return True
except Exception, ex:
return False
if is_clean_ascii(mystr):
<code relying on mystr to be ASCII>
I don't think this is a valid argument because the person who wrote this dual type accepting module was obviously aware about ASCII vs. non ASCII strings and would be aware of encoding and decoding.
I think this evidence is more than enough indication that changing this setting does not lead to any problems in real world codebases the vast majority of the time.
Because you don't always want to have your strings automatically decoded to Unicode, or for that matter your Unicode objects automatically encoded to bytes. Since you are asking for a concrete example, here is one:
Take a WSGI web application; you are building a response by adding the product of an external process to a list, in a loop, and that external process gives you UTF-8 encoded bytes:
results = []
content_length = 0
for somevar in some_iterable:
output = some_process_that_produces_utf8(somevar)
content_length += len(output)
results.append(output)
headers = {
'Content-Length': str(content_length),
'Content-Type': 'text/html; charset=utf8',
}
start_response(200, headers)
return results
That's great and fine and works. But then your co-worker comes along and adds a new feature; you are now providing labels too, and these are localised:
results = []
content_length = 0
for somevar in some_iterable:
label = translations.get_label(somevar)
output = some_process_that_produces_utf8(somevar)
content_length += len(label) + len(output) + 1
results.append(label + '\n')
results.append(output)
headers = {
'Content-Length': str(content_length),
'Content-Type': 'text/html; charset=utf8',
}
start_response(200, headers)
return results
You tested this in English and everything still works, great!
However, the translations.get_label() library actually returns Unicode values and when you switch locale, the labels contain non-ASCII characters.
The WSGI library writes out those results to the socket, and all the Unicode values get auto-encoded for you, since you set setdefaultencoding() to UTF-8, but the length you calculated is entirely wrong. It'll be too short as UTF-8 encodes everything outside of the ASCII range with more than one byte.
All this is ignoring the possibility that you are actually working with data in a different codec; you could be writing out Latin-1 + Unicode, and now you have an incorrect length header and a mix of data encodings.
Had you not used sys.setdefaultencoding() an exception would have been raised and you knew you had a bug, but now your clients are complaining about incomplete responses; there are bytes missing at the end of the page and you don't quite know how that happened.
Note that this scenario doesn't even involve 3rd party libraries that may or may not depend on the default still being ASCII. The sys.setdefaultencoding() setting is global, applying to all code running in the interpreter. How sure are you there are no issues in those libraries involving implicit encoding or decoding?
That Python 2 encodes and decodes between str and unicode types implicitly can be helpful and safe when you are dealing with ASCII data only. But you really need to know when you are mixing Unicode and byte string data accidentally, rather than plaster over it with a global brush and hope for the best.
First of all: Many opponents of changing default enc argue that its dumb because its even changing ascii comparisons
I think its fair to make clear that, compliant with the original question, I see nobody advocating anything else than deviating from Ascii to UTF-8.
The setdefaultencoding('utf-16') example seems to be always just brought forward by those who oppose changing it ;-)
With m = {'a': 1, 'é': 2} and the file 'out.py':
# coding: utf-8
print u'é'
Then:
+---------------+-----------------------+-----------------+
| DEF.ENC | OPERATION | RESULT (printed)|
+---------------+-----------------------+-----------------+
| ANY | u'abc' == 'abc' | True |
| (i.e.Ascii | str(u'abc') | 'abc' |
| or UTF-8) | '%s %s' % ('a', u'a') | u'a a' |
| | python out.py | é |
| | u'a' in m | True |
| | len(u'a'), len(a) | (1, 1) |
| | len(u'é'), len('é') | (1, 2) [*] |
| | u'é' in m | False (!) |
+---------------+-----------------------+-----------------+
| UTF-8 | u'abé' == 'abé' | True [*] |
| | str(u'é') | 'é' |
| | '%s %s' % ('é', u'é') | u'é é' |
| | python out.py | more | 'é' |
+---------------+-----------------------+-----------------+
| Ascii | u'abé' == 'abé' | False, Warning |
| | str(u'é') | Encoding Crash |
| | '%s %s' % ('é', u'é') | Decoding Crash |
| | python out.py | more | Encoding Crash |
+---------------+-----------------------+-----------------+
[*]: Result assumes the same é. See below on that.
While looking at those operations, changing the default encoding in your program might not look too bad, giving you results 'closer' to having Ascii only data.
Regarding the hashing ( in ) and len() behaviour you get the same then in Ascii (more on the results below). Those operations also show that there are significant differences between unicode and byte strings - which might cause logical errors if ignored by you.
As noted already: It is a process wide option so you just have one shot to choose it - which is the reason why library developers should really never ever do it but get their internals in order so that they do not need to rely on python's implicit conversions.
They also need to clearly document what they expect and return and deny input they did not write the lib for (like the normalize function, see below).
=> Writing programs with that setting on makes it risky for others to use the modules of your program in their code, at least without filtering input.
Note: Some opponents claim that def.enc. is even a system wide option (via sitecustomize.py) but latest in times of software containerisation (docker) every process can be started in its perfect environment w/o overhead.
Regarding the hashing and len() behaviour:
It tells you that even with a modified def.enc. you still can't be ignorant about the types of strings you process in your program. u'' and '' are different sequences of bytes in the memory - not always but in general.
So when testing make sure your program behaves correctly also with non Ascii data.
Some say the fact that hashes can become unequal when data values change - although due to implicit conversions the '==' operations remain equal - is an argument against changing def.enc.
I personally don't share that since the hashing behaviour just remains the same as w/o changing it. Have yet to see a convincing example of undesired behaviour due to that setting in a process I 'own'.
All in all, regarding setdefaultencoding("utf-8"): The answer regarding if its dumb or not should be more balanced.
It depends.
While it does avoid crashes e.g. at str() operations in a log statement - the price is a higher chance for unexpected results later since wrong types make it longer into code whose correct functioning depends on a certain type.
In no case it should be the alternative to learning the difference between byte strings and unicode strings for your own code.
Lastly, setting default encoding away from Ascii does not make your life any easier for common text operations like len(), slicing and comparisons - should you assume than (byte)stringyfying everything with UTF-8 on resolves problems here.
Unfortunately it doesn't - in general.
The '==' and len() results are far more complex problem than one might think - but even with the same type on both sides.
W/o def.enc. changed, "==" fails always for non Ascii, like shown in the table. With it, it works - sometimes:
Unicode did standardise around a million symbols of the world and gave them a number - but there is unfortunately NOT a 1:1 bijection between glyphs displayed to a user in output devices and the symbols they are generated from.
To motivate you research this: Having two files, j1, j2 written with the same program using the same encoding, containing user input:
>>> u1, u2 = open('j1').read(), open('j2').read()
>>> print sys.version.split()[0], u1, u2, u1 == u2
Result: 2.7.9 José José False (!)
Using print as a function in Py2 you see the reason: Unfortunately there are TWO ways to encode the same character, the accented 'e':
>>> print (sys.version.split()[0], u1, u2, u1 == u2)
('2.7.9', 'Jos\xc3\xa9', 'Jose\xcc\x81', False)
What a stupid codec you might say but its not the fault of the codec. Its a problem in unicode as such.
So even in Py3:
>>> u1, u2 = open('j1').read(), open('j2').read()
>>> print sys.version.split()[0], u1, u2, u1 == u2
Result: 3.4.2 José José False (!)
=> Independent of Py2 and Py3, actually independent of any computing language you use: To write quality software you probably have to "normalise" all user input. The unicode standard did standardise normalisation.
In Python 2 and 3 the unicodedata.normalize function is your friend.
Real-word example #1
It doesn't work in unit tests.
The test runner (nose, py.test, ...) initializes sys first, and only then discovers and imports your modules. By that time it's too late to change default encoding.
By the same virtue, it doesn't work if someone runs your code as a module, as their initialisation comes first.
And yes, mixing str and unicode and relying on implicit conversion only pushes the problem further down the line.
One thing we should know is
Python 2 use sys.getdefaultencoding() to decode/encode between str and unicode
so if we change default encoding, there will be all kinds of incompatible issues. eg:
# coding: utf-8
import sys
print "你好" == u"你好"
# False
reload(sys)
sys.setdefaultencoding("utf-8")
print "你好" == u"你好"
# True
More examples:
https://pythonhosted.org/kitchen/unicode-frustrations.html
That said, I remember there is some blog suggesting use unicode whenever possible, and only bit string when deal with I/O. I think if your follow this convention, life will be much easier. More solutions can be found:
https://pythonhosted.org/kitchen/unicode-frustrations.html#a-few-solutions
The Code Furies have turned their baleful glares upon me, and it's fallen to me to implement "Secure Transport" as defined by The Direct Project. Whether or not we internally use DNS rather than LDAP for sharing certificates, I'm obviously going to need to set up the former to test against, and that's what's got me stuck. Apparently, an X509 cert needs some massaging to be used in a CERT record, and I'm trying to work out how that's done.
The clearest thing I've found is a script on Videntity's blog, but not being versed in python, I'm hitting a stumbling block. Specifically, this line crashes:
decoded_clean_pk = clean_pk.decode('base64', strict)
since it doesn't seem to like (or rather, to know) whatever 'strict' is supposed to represent. I'm making the semi-educated guess that the line is supposed to decode the base64 data, but I learned from the Debian OpenSSL debacle some years back that blindly diddling with crypto-related code is a Bad Thing(TM).
So I turn the illustrious python wonks on SO to ask if that line might be replaced by this one (with the appropriate import added):
decoded_clean_pk = base64.b64decode(clean_pk)
The script runs after that change, and produces correct-looking output, but I've got enough instinct to know that I can't necessarily trust my instincts here. :)
This line should've work if you would've called like this:
decoded_clean_pk = clean_pk.decode('base64', 'strict')
Notice that strict has to be a string, otherwise python interpreter would try to search for a variable named strict and if it didn't find it or otherwise has other value than: strict, ignore, and replace, it'll probably would've complain about it.
Take a look at this code:
>>>b=base64.b64encode('hello world')
>>>b.decode('base64')
'hello world'
>>>base64.b64decode(b)
'hello world'
Both decode and b64decode works the same when .decode is passed the base64 argument string.
The difference is that str.decode will take a string of bytes as arguments and will return it's Unicode representation depending on the encoding argument you pass as first parameter. In this case, you're telling it to handle a bas64 string so it will do it ok.
To answer your question, both works the same, although b64decode/encode are meant to work only with base64 encodings and str.decode can handle as many encodings as the library is aware of.
For further information take a read at both of the doc sections: decode and b64decode.
UPDATE: Actually, and this is the most important example I guess :) take a look at the source code for encodings/base64_codec.py which is that decode() uses:
def base64_decode(input,errors='strict'):
""" Decodes the object input and returns a tuple (output
object, length consumed).
input must be an object which provides the bf_getreadbuf
buffer slot. Python strings, buffer objects and memory
mapped files are examples of objects providing this slot.
errors defines the error handling to apply. It defaults to
'strict' handling which is the only currently supported
error handling for this codec.
"""
assert errors == 'strict'
output = base64.decodestring(input)
return (output, len(input))
As you may see, it actually uses base64 module to do it :)
Hope this clarify in some way your question.
I am trying to write a small Python 2.x API to support fetching a
job by jobNumber, where jobNumber is provided as an integer.
Sometimes the users provide ajobNumber as an integer literal
beginning with 0, e.g. 037537. (This is because they have been
coddled by R, a language that sanely considers 037537==37537.)
Python, however, considers integer literals starting with "0" to
be OCTAL, thus 037537!=37537, instead 037537==16223. This
strikes me as a blatant affront to the principle of least
surprise, and thankfully it looks like this was fixed in Python
3---see PEP 3127.
But I'm stuck with Python 2.7 at the moment. So my users do this:
>>> fetchJob(037537)
and silently get the wrong job (16223), or this:
>>> fetchJob(038537)
File "<stdin>", line 1
fetchJob(038537)
^
SyntaxError: invalid token
where Python is rejecting the octal-incompatible digit.
There doesn't seem to be anything provided via __future__ to
allow me to get the Py3K behavior---it would have to be built-in
to Python in some manner, since it requires a change to the lexer
at least.
Is anyone aware of how I could protect my users from getting the
wrong job in cases like this? At the moment the best I can think
of is to change that API so it take a string instead of an int.
At the moment the best I can think of is to change that API so it take a string instead of an int.
Yes, and I think this is a reasonable option given the situation.
Another option would be to make sure that all your job numbers contain at least one digit greater than 7 so that adding the leading zero will give an error immediately instead of an incorrect result, but that seems like a bigger hack than using strings.
A final option could be to educate your users. It will only take five minutes or so to explain not to add the leading zero and what can happen if you do. Even if they forget or accidentally add the zero due to old habits, they are more likely to spot the problem if they have heard of it before.
Perhaps you could take the input as a string, strip leading zeros, then convert back to an int?
test = "001234505"
test = int(test.lstrip("0")) # 1234505
Question
It seems that PyWin32 is comfortable with giving null-terminated unicode strings as return values. I would like to deal with these strings the 'right' way.
Let's say I'm getting a string like: u'C:\\Users\\Guest\\MyFile.asy\x00\x00sy'. This appears to be a C-style null-terminated string hanging out in a Python unicode object. I want to trim this bad boy down to a regular ol' string of characters that I could, for example, display in a window title bar.
Is trimming the string off at the first null byte the right way to deal with it?
I didn't expect to get a return value like this, so I wonder if I'm missing something important about how Python, Win32, and unicode play together... or if this is just a PyWin32 bug.
Background
I'm using the Win32 file chooser function GetOpenFileNameW from the PyWin32 package. According to the documentation, this function returns a tuple containing the full filename path as a Python unicode object.
When I open the dialog with an existing path and filename set, I get a strange return value.
For example I had the default set to: C:\\Users\\Guest\\MyFileIsReallyReallyReallyAwesome.asy
In the dialog I changed the name to MyFile.asy and clicked save.
The full path part of the return value was: u'C:\Users\Guest\MyFile.asy\x00wesome.asy'`
I expected it to be: u'C:\\Users\\Guest\\MyFile.asy'
The function is returning a recycled buffer without trimming off the terminating bytes. Needless to say, the rest of my code wasn't set up for handling a C-style null-terminated string.
Demo Code
The following code demonstrates null-terminated string in return value from GetSaveFileNameW.
Directions: In the dialog change the filename to 'MyFile.asy' then click Save. Observe what is printed to the console. The output I get is u'C:\\Users\\Guest\\MyFile.asy\x00wesome.asy'.
import win32gui, win32con
if __name__ == "__main__":
initial_dir = 'C:\\Users\\Guest'
initial_file = 'MyFileIsReallyReallyReallyAwesome.asy'
filter_string = 'All Files\0*.*\0'
(filename, customfilter, flags) = \
win32gui.GetSaveFileNameW(InitialDir=initial_dir,
Flags=win32con.OFN_EXPLORER, File=initial_file,
DefExt='txt', Title="Save As", Filter=filter_string,
FilterIndex=0)
print repr(filename)
Note: If you don't shorten the filename enough (for example, if you try MyFileIsReally.asy) the string will be complete without a null byte.
Environment
Windows 7 Professional 64-bit (no service pack), Python 2.7.1, PyWin32 Build 216
UPDATE: PyWin32 Tracker Artifact
Based on the comments and answers I have received so far, this is likely a pywin32 bug so I filed a tracker artifact.
UPDATE 2: Fixed!
Mark Hammond reported in the tracker artifact that this is indeed a bug. A fix was checked in to rev f3fdaae5e93d, so hopefully that will make the next release.
I think Aleksi Torhamo's answer below is the best solution for versions of PyWin32 before the fix.
I'd say it's a bug. The right way to deal with it would probably be fixing pywin32, but in case you aren't feeling adventurous enough, just trim it.
You can get everything before the first '\x00' with filename.split('\x00', 1)[0].
This doesn't happen on the version of PyWin32/Windows/Python I tested; I don't get any nulls in the returned string even if it's very short. You might investigate if a newer version of one of the above fixes the bug.
ISTR that I had this issue some years ago, then I discovered that such Win32 filename-dialog-related functions return a sequence of 'filename1\0filename2\0...filenameN\0\0', while including possible garbage characters depending on the buffer that Windows allocated.
Now, you might prefer a list instead of the raw return value, but that would be a RFE, not a bug.
PS When I had this issue, I quite understood why one would expect GetOpenFileName to possibly return a list of filenames, while I couldn't imagine why GetSaveFileName would. Perhaps this is considered as API uniformity. Who am I to know, anyway?
I'm trying to write a Python C extension that processes byte strings, and I have something basically working for Python 2.x and Python 3.x.
For the Python 2.x code, near the start of my function, I currently have a line:
if (!PyArg_ParseTuple(args, "s#:in_bytes", &src_ptr, &src_len))
...
I notice that the s# format specifier accepts both Unicode strings and byte strings. I really just want it to accept byte strings and reject Unicode. For Python 2.x, this might be "good enough"--the standard hashlib seems to do the same, accepting Unicode as well as byte strings. However, Python 3.x is meant to clean up the Unicode/byte string mess and not let the two be interchangeable.
So, I'm surprised to find that in Python 3.x, the s format specifiers for PyArg_ParseTuple() still seem to accept Unicode and provide a "default encoded string version" of the Unicode. This seems to go against the principles of Python 3.x, making the s format specifiers unusable in practice. Is my analysis correct, or am I missing something?
Looking at the implementation for hashlib for Python 3.x (e.g. see md5module.c, function MD5_update() and its use of GET_BUFFER_VIEW_OR_ERROUT() macro) I see that it avoids the s format specifiers, and just takes a generic object (O specifier) and then does various explicit type checks using the GET_BUFFER_VIEW_OR_ERROUT() macro. Is this what we have to do?
I agree with you -- it's one of several spots where the C API migration of Python 3 was clearly not designed as carefully and thouroughly as the Python coder-visible parts. I do also agree that probably the best workaround for now is focusing on "buffer views", per that macro -- until and unless something better gets designed into a future Python C API (don't hold your breath waiting for that to happen, though;-).