Noob questions about Python

[Ixiaus]

I have recently (today) just started learning/playing with Python. So
far I am excited and impressed (coming from PHP background).

I have a few questions regarding Python behavior...

val = 'string'
li = list(val)
print li.reverse()

returns nothing, but,

val = 'string'
li = list(val)
li.reverse()
print li

returns what I want. Why does Python do that?

Also I have been playing around with Binary math and noticed that
Python treats:

val = 00110

as the integer 72 instead of returning 00110, why does Python do that?
(and how can I get around it?)

Grateful for any replies!

[Adam Atlas]

On Oct 17, 3:37 pm, Ixiaus <parnel...@comc ast.netwrote:

I have recently (today) just started learning/playing with Python. So
far I am excited and impressed (coming from PHP background).

I have a few questions regarding Python behavior...

val = 'string'
li = list(val)
print li.reverse()

returns nothing, but,

val = 'string'
li = list(val)
li.reverse()
print li

returns what I want. Why does Python do that?

Because list.reverse() modifies a list, it doesn't create and return a
new one.

A common idiom for returning a reversed copy of a list is:
li[::-1]

You can also use the builtin "reversed" -- it doesn't return a list,
but rather a reverse iterator. You can create a list from an iterator
by passing it to the list() initializer, like list(reversed(l i)).

[Bruno Desthuilliers]

Ixiaus a écrit :

I have recently (today) just started learning/playing with Python. So
far I am excited and impressed

Welcome onboard then !-)

(coming from PHP background).

I have a few questions regarding Python behavior...

val = 'string'
li = list(val)
print li.reverse()

returns nothing, but,

val = 'string'
li = list(val)
li.reverse()
print li

returns what I want. Why does Python do that?

list.reverse (like list.sort) is a destructive in-place operation. Not
returning the object is reminder of the destructive nature of the
operation. That's a design choice, whether you agree with it or not
(FWIW, I don't, but I live with it !-)

Note that there's also the reverse() function that returns a reverse
iterator over any sequence, so you could also do:

li = list('allo')
print ''.join(reverse (li))

Also I have been playing around with Binary math and noticed that
Python treats:

val = 00110

as the integer 72 instead of returning 00110, why does Python do that?

Literal integers starting with '0' (zero) are treated as octal. It's a
pretty common convention (like 0x for hexa). FWIW, PHP does just the
same thing.

(and how can I get around it?)

You can't. Python has no literal notation for binary integers so far.
Literal notation for binary ints is not a common feature anyway.

HTH

[Paul Hankin]

On Oct 17, 8:37 pm, Ixiaus <parnel...@comc ast.netwrote:

I have a few questions regarding Python behavior...
as the integer 72 instead of returning 00110, why does Python do that?
(and how can I get around it?)

You can do this:

def bin(x):
return int(x, 2)

val = bin('00110')

--
Paul Hankin

[Neil Cerutti]

On 2007-10-17, Bruno Desthuilliers
<bd************ *****@free.quel quepart.frwrote :

Ixiaus a écrit :

>val = 00110

as the integer 72 instead of returning 00110, why does Python
do that?

Literal integers starting with '0' (zero) are treated as octal.
It's a pretty common convention (like 0x for hexa). FWIW, PHP
does just the same thing.

>(and how can I get around it?)

You can't. Python has no literal notation for binary integers
so far. Literal notation for binary ints is not a common
feature anyway.

You can obtain a practical workaround using the int built-in
function.

>>int('110', 2)

6

--
Neil Cerutti

[Ixiaus]

Thank you for the quick responses.

I did not know that about integer literals beginning with a '0', so
thank you for the explanation. I never really use PHP except for
handling basic forms and silly web stuff, this is why I picked up
Python because I want to teach myself a more powerful and broad
programming language.

With regard to why I asked: I wanted to learn about Binary math in
conjunction with Python, so I wrote a small function that would return
a base 10 number from a binary number. It is nice to know about the
int() function now.

Just for the sake of it, this was the function I came up with:

def bin2dec(val):
li = list(val)
li.reverse()
res = [int(li[x])*2**x for x in range(len(li))]
res.reverse()
print sum(res)

Now that I look at it, I probably don't need that last reverse()
because addition is commutative...

def bin2dec(val):
li = list(val)
li.reverse()
res = [int(li[x])*2**x for x in range(len(li))]
print sum(res)

It basically does the same thing int(string, 2) does.

Thank you for the responses!

[mensanator]

On Oct 17, 4:58 pm, Ixiaus <parnel...@comc ast.netwrote:

Thank you for the quick responses.

I did not know that about integer literals beginning with a '0', so
thank you for the explanation. I never really use PHP except for
handling basic forms and silly web stuff, this is why I picked up
Python because I want to teach myself a more powerful and broad
programming language.

With regard to why I asked: I wanted to learn about Binary math in
conjunction with Python, so I wrote a small function that would return
a base 10 number from a binary number. It is nice to know about the
int() function now.

Just for the sake of it, this was the function I came up with:

def bin2dec(val):
li = list(val)
li.reverse()
res = [int(li[x])*2**x for x in range(len(li))]
res.reverse()
print sum(res)

Now that I look at it, I probably don't need that last reverse()
because addition is commutative...

def bin2dec(val):
li = list(val)
li.reverse()
res = [int(li[x])*2**x for x in range(len(li))]
print sum(res)

It basically does the same thing int(string, 2) does.

Thank you for the responses!

You could also get ahold of the gmpy module. You get conversion
to binary and also some useful other binary functions as shown
below:

# the Collatz Conjecture in binary

import gmpy

n = 27
print '%4d %s' % (n,gmpy.digits( n,2).zfill(16))

sv = [] # sequence vector, blocks of contiguous LS 0's

while n != 1:
old_n = n
n = 3*n + 1 # result always even
f = gmpy.scan1(n,0) # find least significant 1 bit
n >>= f # remove LS 0's in one fell swoop
sv.append(f) # record f sequence
PopC = gmpy.popcount(n ) # count of 1 bits
HamD = gmpy.hamdist(n, old_n) # bits changed
print '%4d %s' % (n,gmpy.digits( n,2).zfill(16)) ,
print 'PopC:%2d HamD:%2d' % (PopC,HamD)

print sv

## 27 000000000001101 1
## 41 000000000010100 1 PopC: 3 HamD: 3
## 31 000000000001111 1 PopC: 5 HamD: 4
## 47 000000000010111 1 PopC: 5 HamD: 2
## 71 000000000100011 1 PopC: 4 HamD: 3
## 107 000000000110101 1 PopC: 5 HamD: 3
## 161 000000001010000 1 PopC: 3 HamD: 4
## 121 000000000111100 1 PopC: 5 HamD: 4
## 91 000000000101101 1 PopC: 5 HamD: 2
## 137 000000001000100 1 PopC: 3 HamD: 4
## 103 000000000110011 1 PopC: 5 HamD: 6
## 155 000000001001101 1 PopC: 5 HamD: 6
## 233 000000001110100 1 PopC: 5 HamD: 4
## 175 000000001010111 1 PopC: 6 HamD: 3
## 263 000000010000011 1 PopC: 4 HamD: 4
## 395 000000011000101 1 PopC: 5 HamD: 3
## 593 000000100101000 1 PopC: 4 HamD: 7
## 445 000000011011110 1 PopC: 7 HamD: 7
## 167 000000001010011 1 PopC: 5 HamD: 4
## 251 000000001111101 1 PopC: 7 HamD: 4
## 377 000000010111100 1 PopC: 6 HamD: 3
## 283 000000010001101 1 PopC: 5 HamD: 3
## 425 000000011010100 1 PopC: 5 HamD: 4
## 319 000000010011111 1 PopC: 7 HamD: 4
## 479 000000011101111 1 PopC: 8 HamD: 3
## 719 000000101100111 1 PopC: 7 HamD: 3
##1079 000001000011011 1 PopC: 6 HamD: 7
##1619 000001100101001 1 PopC: 6 HamD: 4
##2429 000010010111110 1 PopC: 8 HamD: 8
## 911 000000111000111 1 PopC: 7 HamD: 7
##1367 000001010101011 1 PopC: 7 HamD: 6
##2051 000010000000001 1 PopC: 3 HamD: 6
##3077 000011000000010 1 PopC: 4 HamD: 3
## 577 000000100100000 1 PopC: 3 HamD: 5
## 433 000000011011000 1 PopC: 5 HamD: 6
## 325 000000010100010 1 PopC: 4 HamD: 5
## 61 000000000011110 1 PopC: 5 HamD: 5
## 23 000000000001011 1 PopC: 4 HamD: 3
## 35 000000000010001 1 PopC: 3 HamD: 3
## 53 000000000011010 1 PopC: 4 HamD: 3
## 5 000000000000010 1 PopC: 2 HamD: 2
## 1 000000000000000 1 PopC: 1 HamD: 1
##[1, 2, 1, 1, 1, 1, 2, 2, 1, 2, 1, 1, 2,
## 1, 1, 1, 2, 3, 1, 1, 2, 1, 2, 1, 1, 1,
## 1, 1, 3, 1, 1, 1, 4, 2, 2, 4, 3, 1, 1,
## 5, 4]

[Michele Simionato]

On Oct 17, 5:58 pm, Ixiaus <parnel...@comc ast.netwrote:

def bin2dec(val):
li = list(val)
li.reverse()
res = [int(li[x])*2**x for x in range(len(li))]
print sum(res)

It basically does the same thing int(string, 2) does.

Thank you for the responses!

BTW, here is the reverse function dec2bin, so that you can
bang your head on it for a while ;)

def baseN(number, N=2):
"""

>>baseN(9, 2)

'1001'
"""
assert 2 <= N <= 10
assert isinstance(numb er, int) and number >= 0
b = []
while number:
b.append(str(nu mber % N))
number /= N
return ''.join(reverse d(b))

Michele Simionato

[Stargaming]

On Wed, 17 Oct 2007 22:05:36 +0200, Bruno Desthuilliers wrote:
[snip]

>
Note that there's also the reverse() function that returns a reverse
iterator over any sequence, so you could also do:

li = list('allo')
print ''.join(reverse (li))

Note this certainly should've been `reversed()`, with a trailing 'd'.

[Bruno Desthuilliers]

Stargaming a écrit :

On Wed, 17 Oct 2007 22:05:36 +0200, Bruno Desthuilliers wrote:
[snip]

>Note that there's also the reverse() function that returns a reverse
iterator over any sequence, so you could also do:

li = list('allo')
print ''.join(reverse (li))

Note this certainly should've been `reversed()`, with a trailing 'd'.

2-0 for Stargaming. I'll have to either change glasses, buy a new
keyboard (this one is obviously broken), or learn to both use a keyboard
and re-read what I post.