scanf in python

[AMD]

Hello,

I often need to parse strings which contain a mix of characters,
integers and floats, the C-language scanf function is very practical for
this purpose.
I've been looking for such a feature and I have been quite surprised to
find that it has been discussed as far back as 2001 but never
implemented. The recommended approach seems to be to use split and then
atoi or atof or to use regex and then atoi and atof. Both approaches
seem to be a lot less natural and much more cumbersome than scanf. If
python already has a % string operator that behaves like printf, why not
implement either a %% or << string operator to behave like scanf, use
could be like the followng:

a, b, c = "%d %f %5c" %% "1 2.0 abcde"

or

a, b, c = "%d %f %5c" << "1 2.0 abcde"

%% is closer to the % operator

<< seems more intuitive to me

either of this methods seems to me much simpler than:

lst = "1 2;0 abcde".split()
a = int(lst[0])
b = float(lst[1])
c = lst[2]

or even worse when using regular expressions to parse such simple input.

I like python because it is concise and easy to read and I really think
it could use such an operator.

I know this has been discussed before and many times, but all previous
threads I found seem to be dead and I would like to invite further
investigation of this topic.

Cheers,

André M. Descombes

 

[Diez B. Roggisch]

Hello,

I often need to parse strings which contain a mix of characters,
integers and floats, the C-language scanf function is very practical for
this purpose.
I've been looking for such a feature and I have been quite surprised to
find that it has been discussed as far back as 2001 but never
implemented. The recommended approach seems to be to use split and then
atoi or atof or to use regex and then atoi and atof. Both approaches
seem to be a lot less natural and much more cumbersome than scanf. If
python already has a % string operator that behaves like printf, why not
implement either a %% or << string operator to behave like scanf, use
could be like the followng:

a, b, c = "%d %f %5c" %% "1 2.0 abcde"

or

a, b, c = "%d %f %5c" << "1 2.0 abcde"

%% is closer to the % operator

<< seems more intuitive to me

either of this methods seems to me much simpler than:

lst = "1 2;0 abcde".split()
a = int(lst[0])
b = float(lst[1])
c = lst[2]

or even worse when using regular expressions to parse such simple input.

I like python because it is concise and easy to read and I really think
it could use such an operator.

I know this has been discussed before and many times, but all previous
threads I found seem to be dead and I would like to invite further
investigation of this topic.

I'm pretty certain python won't grow an additional operator for this.
Yet you are free to create a scanf-implementation as 3rd-party-module.

IMHO the usability of the approach is very limited though. First of all,
the need to capture more than one input token is *very* seldom - nearly
all commandline-tools I know that do require interactive user-input
(like the linux kernel config tool) do so by providing either
line-by-line value entry (including defaults, something you can't do
with your approach), or even dialog-centric value entry with curses.

So - I doubt you will gather much momentum on this. Good luck though.


Diez

 

[AMD]

I'm pretty certain python won't grow an additional operator for this.
Yet you are free to create a scanf-implementation as 3rd-party-module.

IMHO the usability of the approach is very limited though. First of all,
the need to capture more than one input token is *very* seldom - nearly
all commandline-tools I know that do require interactive user-input
(like the linux kernel config tool) do so by providing either
line-by-line value entry (including defaults, something you can't do
with your approach), or even dialog-centric value entry with curses.

So - I doubt you will gather much momentum on this. Good luck though.


Diez

Actually it is quite common, it is used for processing of files not for
reading parameters. You can use it whenever you need to read a simple
csv file or fixed format file which contains many lines with several
fields per line.
The advantage of the approach is that it combines the parsing and
conversion of the fields into one operation.
Another advantage of using simple formatting strings is that it allows
for easy translation of these lines, just like you have with the %
operator for output. I don't see why python can have an operator for
output but it can't have one for input, it's just not symmetrical.
I don´t see why you can't use this method for line-by-line value entry,
just add \n between your %s or %d.
The method is quite versatile and much simpler than regular expressions
plus conversion afterwards.

André

 

[Robert Kern]

Hello,

I often need to parse strings which contain a mix of characters,
integers and floats, the C-language scanf function is very practical for
this purpose.
I've been looking for such a feature and I have been quite surprised to
find that it has been discussed as far back as 2001 but never
implemented.

The second Google hit is a pure Python implementation of scanf.

http://hkn.eecs.berkeley.edu/~dyoo/python/scanf/

--
Robert Kern

"I have come to believe that the whole world is an enigma, a harmless enigma
that is made terrible by our own mad attempt to interpret it as though it had
an underlying truth."
-- Umberto Eco

 

[AMD]

Robert Kern a écrit :

The second Google hit is a pure Python implementation of scanf.

http://hkn.eecs.berkeley.edu/~dyoo/python/scanf/

Hi Robert,

I had seen this pure python implementation, but it is not as fast or as
elegant as would be an implementation written in C directly within
python with no need for import.

Cheers,

André

 

[Fredrik Lundh]

I had seen this pure python implementation, but it is not as fast or as
elegant as would be an implementation written in C directly within
python with no need for import.

maybe you should wait with disparaging comments about how Python is not
what you want it to be until you've learned the language?

</F>

 

[AMD]

maybe you should wait with disparaging comments about how Python is not
what you want it to be until you've learned the language?

</F>

Hello Fredrik,

I didn't think my comment would offend anyone, I'm sorry if it did. I
have been developping in Python for about 5 years, my company uses
Python as a scripting language for all of its products. We use Jython
for our server products. I think I know it pretty well by now. So I
think I have earned the right to try to suggest improvements to the
language or at least intelligent discussion of new features without need
for offensive comments, don't you think?

André

 

[Lawrence D'Oliveiro]

Actually it is quite common, it is used for processing of files not for
reading parameters. You can use it whenever you need to read a simple
csv file or fixed format file which contains many lines with several
fields per line.

I do that all the time, in Python and C++, but I've never felt the need for
a scanf-type function.

For reading delimited fields in Python, you can use .split string method.

 

[AMD]

I do that all the time, in Python and C++, but I've never felt the need for
a scanf-type function.

I agree scanf is not a must have function but rather a nice to have
function.

For reading delimited fields in Python, you can use .split string method.

Yes, that is what I use right now, but I still have to do the conversion
to integers, floats, dates as several separate steps. What is nice about
the scanf function is that it is all done on the same step. Exactly like
when you use % to format a string and you pass it a dictionary, it does
all the conversions to string for you.

Cheers,

André

 

[Fredrik Lundh]

Yes, that is what I use right now, but I still have to do the conversion
to integers, floats, dates as several separate steps. What is nice about
the scanf function is that it is all done on the same step. Exactly like
when you use % to format a string and you pass it a dictionary, it does
all the conversions to string for you.

You're confusing surface syntax with processing steps. If you want to
do things on one line, just add a suitable helper to take care of the
processing. E.g. for whitespace-separated data:
.... return tuple(f(v) for (f, v) in zip(types, s.split()))
....(1, 2, 3.0)

This has the additional advantage that it works with any data type that
provides a way to convert from string to that type, not just a small
number of built-in types. And you can even pass in your own local
helper, of course:
.... return int(n) * "!"
....(1, 2.0, '!!!')

If you're reading multiple columns of the same type, you might as well
inline the whole thing:

data = map(int, line.split())

For other formats, replace the split with slicing or a regexp. Or use a
ready-made module; there's hardly every any reason to read standard CSV
files by hand when you can just do "import csv", for example.

Also note that function *creation* is relatively cheap in Python, and
since "def" is an executable statement, you can create them pretty much
anywhere; if you find that need a helper somewhere in your code, just
put it there. The following is a perfectly valid pattern:

def myfunc(...):

def myhelper(...):
...

myhelper(...)
myhelper(...)

for line in open(file):
myhelper(...)

(I'd say knowing when and how to abstract things away into a local
helper is an important step towards full Python fluency -- that is, the
point where you're able to pack "a lot of action in a small amount of
clear code" most of the time.)

</F>

 

[AMD]

Thanks Fredrik,

very nice examples.

André

You're confusing surface syntax with processing steps. If you want to
do things on one line, just add a suitable helper to take care of the
processing. E.g. for whitespace-separated data:

... return tuple(f(v) for (f, v) in zip(types, s.split()))
...
(1, 2, 3.0)

This has the additional advantage that it works with any data type that
provides a way to convert from string to that type, not just a small
number of built-in types. And you can even pass in your own local
helper, of course:

... return int(n) * "!"
...
(1, 2.0, '!!!')

If you're reading multiple columns of the same type, you might as well
inline the whole thing:

data = map(int, line.split())

For other formats, replace the split with slicing or a regexp. Or use a
ready-made module; there's hardly every any reason to read standard CSV
files by hand when you can just do "import csv", for example.

Also note that function *creation* is relatively cheap in Python, and
since "def" is an executable statement, you can create them pretty much
anywhere; if you find that need a helper somewhere in your code, just
put it there. The following is a perfectly valid pattern:

def myfunc(...):

def myhelper(...):
...

myhelper(...)
myhelper(...)

for line in open(file):
myhelper(...)

(I'd say knowing when and how to abstract things away into a local
helper is an important step towards full Python fluency -- that is, the
point where you're able to pack "a lot of action in a small amount of
clear code" most of the time.)

</F>

 

[Lawrence D'Oliveiro]

I agree scanf is not a must have function but rather a nice to have
function.

I've never felt that scanf would be "nice" to have. Not in Python, not in
C++.

 

[tutufan]

Hello Fredrik,

I didn't think my comment would offend anyone [...]

I doubt that it offended anyone else. Having been the recipient of a
few F-bombs myself, I'd just let it go by...

Mike

 

[castironpi]

Hello Fredrik,

I didn't think my comment would offend anyone [...]

I doubt that it offended anyone else.  Having been the recipient of a
few F-bombs myself, I'd just let it go by...

Mike

The regular expression module (re) should be pretty handy at this.
This may not be a typical case, but:

re.match( r"([0-9]+) ([0-9]*.?[0-9]+) (.{1,5})", '1 2.02 abcde' ).groups( )

('1', '2.02', 'abcde')

The float catcher doesn't catch "2." If you need that to work or
other classes, speak up.