[Tutor] Why is an OrderedDict not sliceable?
Hi,
Like the subject says: Why is an OrderedDict not sliceable? (From the
collections library). Was that an intentional omission, or a mistake? [1]
Background: I do not use OrderedDict very often, but I thought I could use it
to look up street and city names using postcodes ([0-9]{4} [a-z]{2} format). I
needed it to be ordered because I also wanted to be able to use bisect, which
is needed when the postcode letters are missing. In short: a fast dict lookup
for complete postcodes and less fast bisect lookup for in complete postcodes.
[1] http://stackoverflow.com/questions/30975339/slicing-a-python-ordereddict
Thanks!
Albert-Jan
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[Tutor] import cv, not cv2
After successfully installing dlib and cv2, I have now been asked to install the cv library on Ubuntu. >From what I have read, it seems OpenCV has discontinued support for cv and only cv2 is used now. Am I correct? Regardless, does anyone know how I can get this library? I apologize in advance if my question is foolish. Thank you! Cheers! ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] import cv, not cv2
On 20/01/16 10:55, Rheeya Uppaal wrote: > After successfully installing dlib and cv2, I have now been asked to > install the cv library on Ubuntu. > > From what I have read, it seems OpenCV has discontinued support for cv and > only cv2 is used now. Am I correct? Regardless, does anyone know how I can > get this library? This isn't really a Python language or standard library question. You should try asking on the openCV support forum: http://answers.opencv.org/questions/ They should be able to help you. -- Alan G Author of the Learn to Program web site http://www.alan-g.me.uk/ http://www.amazon.com/author/alan_gauld Follow my photo-blog on Flickr at: http://www.flickr.com/photos/alangauldphotos ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Why is an OrderedDict not sliceable?
On Wed, Jan 20, 2016 at 01:33:17PM +, Albert-Jan Roskam wrote: > Hi, > > Like the subject says: Why is an OrderedDict not sliceable? (From the > collections library). Was that an intentional omission, or a mistake? > [1] Because slicing a dict makes no sense. A dict is a mapping, not a sequence. d = OrderedDict() d["cow"] = "a" d["fox"] = "b" d["ape"] = "c" d["dog"] = "d" d["bee"] = "e" I can do a dict lookup on a key: d["cow"] What would a slice even mean? d[1:4] but 1, 2, 3 are not keys of the dict. -- Steve ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
[Tutor] Why is the name "self" optional instead of mandatory?
I'm whizzing along in "Python Crash Course" and am in the chapter on
classes. Just to satisfy my suspicion that "self" is just a
placeholder for creating an object instance, I tried the following:
>>> class Dog(object):
def __init__(this, name, age):
this.name = name
this.age = age
def bark(this):
print("Woof! Woof! Grrr!!!")
def whoami(this):
print("My name is", this.name.title(), "and I am",
this.age, "years old.")
>>> mydog = Dog('Spotty', 50)
>>> mydog.bark()
Woof! Woof! Grrr!!!
>>> mydog.whoami()
My name is Spotty and I am 50 years old.
And just to be really silly:
>>> class Cat(object):
def __init__(MEO, name, age):
MEO.name = name
MEO.age = age
def happy_cat(MEO):
print("Zzzz ... pu ... ")
def whoami(MEO):
print("My name is", MEO.name, "and I am", MEO.age,
"years old. Now leave me be! I'm very sleepy!!!")
>>> mycat = Cat('Callie', 7)
>>> mycat.happy_cat()
Zzzz ... pu ...
>>> mycat.whoami()
My name is Callie and I am 7 years old. Now leave me be! I'm very sleepy!!!
So I really only have one question: Why not make Python's
*traditional* name, "self", mandatory? Why give the programmer this
kind of choice? [OK, that was two questions.]
TIA!
--
boB
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[Tutor] Why do I not get an error when I mistakenly type "humdrum.sigh_strenght" instead of the correct "humdrum.sigh_strength"?
My intent was to deliberately introduce an error into my class definition:
>>> class Hmm(object):
def __init__(self, sigh_type, sigh_strength):
self.sigh_type = sigh_type
self.sigh_strength = sigh_strength
def snort(self):
if self.sigh_strength == 'low':
print("snort")
elif self.sigh_strength == 'med':
print("Snort!")
elif self.sigh_strenght == 'high':
print("SNORT!!!")
else:
print("Hmm...")
def sigh():
if self.sigh_type == 'quiet':
print("pss")
elif self.sigh_type == 'annoying':
print("Whoosh!")
elif self.sigh_type == 'loud':
print("HEAVY SIGH!!!")
else:
print("HMM!!!")
I omitted "self" from the sigh() method to see what would happen plus
some other things.
>>> humdrum = Hmm('quiet', 'low')
>>> humdrum.snort()
snort
>>> humdrum.sigh_strength = 'med'
>>> humdrum.snort()
Snort!
>>> humdrum.sigh_strenght = 'high'
>>> humdrum.snort()
Snort!
At this point I wondered why my output was not "SNORT!!!". Then I
noticed my typo. But now I wonder why I did not get an error from
this typo?
>>> humdrum.sigh_strength = 'high'
>>> humdrum.snort()
SNORT!!!
>>> humdrum.sigh()
Traceback (most recent call last):
File "", line 1, in
humdrum.sigh()
TypeError: sigh() takes 0 positional arguments but 1 was given
This was my original point in doing all of this, to see what would
result if I omitted "self". I am pretty sure the error is because the
object instance gets automatically passed to the sigh() method, but by
leaving the "self" parameter out in the method definition, I have a
mismatch between what was defined (0 parameters) and what was passed
to the method (1 argument).
>>> humdrum.sigh_strenght
'high'
But what about this? It seems like I can use the humdrum arguments
outside of the Hmm class and merrily define new variables. Why is
this? Is this potentially useful behavior?
--
boB
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Re: [Tutor] Why do I not get an error when I mistakenly type "humdrum.sigh_strenght" instead of the correct "humdrum.sigh_strength"?
On 20Jan2016 22:34, boB Stepp wrote:
My intent was to deliberately introduce an error into my class definition:
class Hmm(object):
def __init__(self, sigh_type, sigh_strength):
self.sigh_type = sigh_type
self.sigh_strength = sigh_strength
def snort(self):
if self.sigh_strength == 'low':
print("snort")
elif self.sigh_strength == 'med':
print("Snort!")
elif self.sigh_strenght == 'high':
print("SNORT!!!")
else:
print("Hmm...")
def sigh():
if self.sigh_type == 'quiet':
print("pss")
elif self.sigh_type == 'annoying':
print("Whoosh!")
elif self.sigh_type == 'loud':
print("HEAVY SIGH!!!")
else:
print("HMM!!!")
I omitted "self" from the sigh() method to see what would happen plus
some other things.
Well... You've bound a function accepting no arguments to the "sigh" attribute
of the class. Legal. Nonsensical perhaps, but legal.
humdrum = Hmm('quiet', 'low')
humdrum.snort()
snort
humdrum.sigh_strength = 'med'
humdrum.snort()
Snort!
humdrum.sigh_strenght = 'high'
humdrum.snort()
Snort!
At this point I wondered why my output was not "SNORT!!!". Then I
noticed my typo. But now I wonder why I did not get an error from
this typo?
Because your "if" statement matched the "med". So it never tried to look up
"self.sigh_strenght".
humdrum.sigh_strength = 'high'
humdrum.snort()
SNORT!!!
Again, as you expected, yes?
humdrum.sigh()
Traceback (most recent call last):
File "", line 1, in
humdrum.sigh()
TypeError: sigh() takes 0 positional arguments but 1 was given
This was my original point in doing all of this, to see what would
result if I omitted "self". I am pretty sure the error is because the
object instance gets automatically passed to the sigh() method, but by
leaving the "self" parameter out in the method definition, I have a
mismatch between what was defined (0 parameters) and what was passed
to the method (1 argument).
Correct.
humdrum.sigh_strenght
'high'
But what about this? It seems like I can use the humdrum arguments
outside of the Hmm class and merrily define new variables. Why is
this? Is this potentially useful behavior?
"humdrum" is just an object. You can assign attibutes to it at any time.
The code executing inside the class is no different to the code outside the
class; Python is a dynamic language and you can do this stuff at any time.
It isn't _specificly_ useful to assign an attribute long after its normal
initialisation, but it can be quite useful. But consider your initialiser:
def __init__(self, sigh_type, sigh_strength):
self.sigh_type = sigh_type
self.sigh_strength = sigh_strength
By the time __init__ is called, the object already exists with a type/class and
everything. All __init__ is doing is what you find unexpected later; defining
new attribute values not there before. The only thing special about __init__ is
that it is called automatically after an object is created. But that is all.
This is not a static language, and __init__ is not defining what
fields/attributes the object possesses. It is merely setting some of them. It
is executable code, not a static type definition.
Cheers,
Cameron Simpson
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Re: [Tutor] Why is the name "self" optional instead of mandatory?
On 20Jan2016 21:42, boB Stepp wrote: I'm whizzing along in "Python Crash Course" and am in the chapter on classes. Just to satisfy my suspicion that "self" is just a placeholder for creating an object instance, No, it is a placeholder for a _preexiting_ object instance. [...] So I really only have one question: Why not make Python's *traditional* name, "self", mandatory? Why give the programmer this kind of choice? [OK, that was two questions.] Why make it mandatory? What benefit would it bring? Remember, one can write nonsense or impossible to read gibberish in any language; most people don't try to. So to repeat my question: why make it mandatory? Cheers, Cameron Simpson ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Why do I not get an error when I mistakenly type "humdrum.sigh_strenght" instead of the correct "humdrum.sigh_strength"?
humdrum.sigh_strenght = 'high' humdrum.snort() > Snort! > > At this point I wondered why my output was not "SNORT!!!". Then I > noticed my typo. But now I wonder why I did not get an error from > this typo? Hi boB, Just to be explicit: you are pointing out that: humdrum.sigh_strenght = 'high' was a typo, and it would have been nice if it could be caught as an error. If that's the case, then yes, I agree. Unfortunately, this isn't a runtime error because Python allows us to add arbitrary attributes to objects: it's a deliberate design feature. One justification for this appears to be similar to that of dynamic typing: if we have good unit tests, we should be able to catch these problems with our unit tests, so why add a restriction? (As a personal note: I'm not entirely convinced of the merit of this particular freedom. Misspelling is a really easy mistake to make, and I want the language to catch my simple mistakes, if it's not too onerous to do so.) That being said, there are tools available that should be able to catch many of these problems before runtime. http://www.pylint.org/ http://pychecker.sourceforge.net/ These should raise warnings if you run it on your program. You might want to take a look at these! Furthermore, there actually *is* a way to turn it into a runtime error, though this is probably not beginner material. If you're interested, see material on "__slots__". https://docs.python.org/3/reference/datamodel.html#slots. I don't think it's intended to be used to catch typos though: its main purpose is to reduce memory usage, and it's use to catch misspellings is incidental. Hope this helps! ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Why is the name "self" optional instead of mandatory?
> So I really only have one question: Why not make Python's > *traditional* name, "self", mandatory? Why give the programmer this > kind of choice? [OK, that was two questions.] There are situations where it might not be mandatory. This is definitely not beginner Python material, so I don't want to delve into it too deeply. Roughly: we can do things like "metaclass" programming where your programs modify the rules that construct classes. In that way, it's possible to do fairly crazy things. We might even be able to make it optional to say "self" all the time, if we modify the rules radically enough. If you are really interested in this, see: https://jakevdp.github.io/blog/2012/12/01/a-primer-on-python-metaclasses/ for an primer. Basically, Python does almost everything at run-time. The sorts of things we'd expect to be static properties aren't static in Python. Other languages tend to lock things down a bit more firmly, or at the very least, do more things at compile-time. Python programmers have enormous flexibility at run-time, at the cost of making it really easy to make mistakes that aren't caught until run-time. ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Why is the name "self" optional instead of mandatory?
boB Stepp writes: > So I really only have one question: Why not make Python's > *traditional* name, "self", mandatory? Why give the programmer this > kind of choice? For the same reason that four-space indentation is not mandatory, yet anyone who chooses a different indentation size needs to produce good reasons why. In both cases: Because nothing is broken, nor made especially ambiguous, by choosing differently. And because the Python community of programmers can be relied upon to enforce the convention, as with most other conventions. -- \ “When I was a little kid we had a sand box. It was a quicksand | `\ box. I was an only child... eventually.” —Steven Wright | _o__) | Ben Finney ___ Tutor maillist - Tutor@python.org To unsubscribe or change subscription options: https://mail.python.org/mailman/listinfo/tutor
Re: [Tutor] Why do I not get an error when I mistakenly type "humdrum.sigh_strenght" instead of the correct "humdrum.sigh_strength"?
boB Stepp writes:
> My intent was to deliberately introduce an error into my class definition:
>
> >>> class Hmm(object):
> def __init__(self, sigh_type, sigh_strength):
> self.sigh_type = sigh_type
> self.sigh_strength = sigh_strength
> def snort(self):
> if self.sigh_strength == 'low':
> print("snort")
> elif self.sigh_strength == 'med':
> print("Snort!")
> elif self.sigh_strenght == 'high':
> print("SNORT!!!")
> else:
> print("Hmm...")
> def sigh():
> if self.sigh_type == 'quiet':
> print("pss")
> elif self.sigh_type == 'annoying':
> print("Whoosh!")
> elif self.sigh_type == 'loud':
> print("HEAVY SIGH!!!")
> else:
> print("HMM!!!")
>
> I omitted "self" from the sigh() method to see what would happen
What would happen, when? At the time of class definition, there are no
errors that I can see.
Python doesn't run every possible branch of your code ahead of time,
just to find out what it will do. Most errors will be discovered only by
encountering the combination of inputs that will trigger them at run
time.
For this and other reasons, it is highly recommended to develop unit
tests with your code, to make explicit assertions about exactly what
will trigger each branch of the code.
This, in turn, will lead to making your code simpler and less-branching
:-)
--
\ “Do unto others twenty-five percent better than you expect them |
`\ to do unto you. (The twenty-five percent is [to correct] for |
_o__)error.)” —Linus Pauling's Golden Rule |
Ben Finney
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