cedric's bazaar - cs_lang:python:things-to-know

comprehensions

anonymous@undisclosed.example.com (Anonymous) — Sun, 12 Dec 2010 17:34:08 +0000

List Comprehensions

>>> a_list = [1, 9, 8, 4]
>>> 
>>> [elem * 2 for elem in a_list]
[2, 18, 16, 8]
>>> a_list
[1, 9, 8, 4]
>>> a_list = [elem * 2 for elem in a_list]
>>> a_list
[2, 18, 16, 8]

>>> [a for a in range(0,99) if a % 2 == 0]
[0, 2, 4, 6, 8, 10, 12, 14, 16, 18, 20, 22, 24, 26, 28, 30, 32, 34, 36, 38, 40, 42, 44, 46, 48, 50, 52, 54, 56, 58, 60, 62, 64, 66, 68, 70, 72, 74, 76, 78, 80, 82, 84, 86, 88, 90, 92, 94, 96, 98]
>>>
>>> [a % 2 == 0 for a in range(0,99)]
[True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True, False, True]

Dictionary Comprehensions

date-and-time

anonymous@undisclosed.example.com (Anonymous) — Wed, 20 Jun 2012 09:14:09 +0000

import time
 
#pattern = "%a %b %d %H:%M:%S  %Y"
pattern = '%Y-%m-%d %H:%M:%S'
date_time = '2007-02-05 16:15:18'
 
epoch = int(time.mktime(time.strptime(date_time, pattern)))
print epoch

def delta_time(dt, delta, fmt='%Y-%m-%d %H:%M:%S'):
    """
    Adds a time delta to a datetime object with a specific format.
 
    Example of call:
    >>> new_dt = delta_time('2012-10-27 12:05:54', datetime.timedelta(minutes=45))
    >>> print("%s:%s:%s" % (new_dt.hour, new_dt.minute, new_dt.second))
    12:50:54
    """
    return datetime.datetime(*time.strptime(dt, fmt)[:6]) + delta

exceptions

anonymous@undisclosed.example.com (Anonymous) — Sun, 12 Dec 2010 16:29:04 +0000

>>> a
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
NameError: name 'a' is not defined

>>> try:
...   a 
... except:
...   print("'a' is not defined.")
... 
'a' is not defined.

files

anonymous@undisclosed.example.com (Anonymous) — Sun, 12 Dec 2010 18:21:02 +0000

Reading a file

Line by line

>>> for line in open("./dico.txt", "r"):
...   print(line)
... 
ABACA~
 
ABACULE~
 
ABAISSABLE~
 
ABAISSAIENT~
 
ABAISSAIS~
.
.
.

Classic method

>>> a_file = None
>>> try:              
...   a_file = open("./dico.txt", "r")
...   text = a_file.read()
... except:
...   print("Error")
... finally:
...   if a_file != None:
...     a_file.close()
... 
>>> 
>>> a_file
<_io.TextIOWrapper name='./dico.txt' encoding='UTF-8'>
>>> a_file.name
'./dico.txt'

The with statement

The better solution. No need to use try: … except: ….
Forget the previous method.

>>> with open("./dico.txt", "r") as f:
...   text = f.read()
>>>
>>> len(text)
2664170
>>> print(text)
.
.
.
ZYGOMORPHE~
ZYGOMYCETES~
ZYGOMYCETE~
ZYGOPETALE~
ZYGOTE~
ZYMASE~
ZYMOTECHNIE~
ZYMOTIQUE~
ZYTHON~
ZYTHUM~
Z~

This code calls open(), but it never calls a_file.close(). The with statement starts a code block, like an if statement or a for loop. Inside this code block, you can use the variable a_file as the stream object returned from the call to open(). All the regular stream object methods are available — seek(), read(), whatever you need. When the with block ends, Python calls a_file.close() automatically.

geographic

anonymous@undisclosed.example.com (Anonymous) — Wed, 20 Jun 2012 09:21:04 +0000

def Deg2Rad(x):
    """
    Degrees to radians.
    """
    return x * (math.pi/180)
 
def Rad2Deg(x):
    """
    Radians to degress.
    """
    return x * (180/math.pi)
 
def CalcRad(lat):
    """
    Radius of curvature in meters at specified latitude.
    """
    a = 6378.137
    e2 = 0.081082 * 0.081082
    # the radius of curvature of an ellipsoidal Earth in the plane of a
    # meridian of latitude is given by
    #
    # R' = a * (1 - e^2) / (1 - e^2 * (sin(lat))^2)^(3/2)
    #
    # where a is the equatorial radius,
    # b is the polar radius, and
    # e is the eccentricity of the ellipsoid = sqrt(1 - b^2/a^2)
    #
    # a = 6378 km (3963 mi) Equatorial radius (surface to center distance)
    # b = 6356.752 km (3950 mi) Polar radius (surface to center distance)
    # e = 0.081082 Eccentricity
    sc = math.sin(Deg2Rad(lat))
    x = a * (1.0 - e2)
    z = 1.0 - e2 * sc * sc
    y = pow(z, 1.5)
    r = x / y
 
    r = r * 1000.0      # Convert to meters
    return r
 
 
def EarthDistance((lat1, lon1), (lat2, lon2)):
    """
    Distance in meters between two points specified in degrees.
    """
    x1 = CalcRad(lat1) * math.cos(Deg2Rad(lon1)) * math.sin(Deg2Rad(90-lat1))
    x2 = CalcRad(lat2) * math.cos(Deg2Rad(lon2)) * math.sin(Deg2Rad(90-lat2))
    y1 = CalcRad(lat1) * math.sin(Deg2Rad(lon1)) * math.sin(Deg2Rad(90-lat1))
    y2 = CalcRad(lat2) * math.sin(Deg2Rad(lon2)) * math.sin(Deg2Rad(90-lat2))
    z1 = CalcRad(lat1) * math.cos(Deg2Rad(90-lat1))
    z2 = CalcRad(lat2) * math.cos(Deg2Rad(90-lat2))
    a = (x1*x2 + y1*y2 + z1*z2)/pow(CalcRad((lat1+lat2)/2), 2)
    # a should be in [1, -1] but can sometimes fall outside it by
    # a very small amount due to rounding errors in the preceding
    # calculations (this is prone to happen when the argument points
    # are very close together).  Thus we constrain it here.
    if abs(a) > 1:
        a = 1
    elif a < -1:
        a = -1
    return CalcRad((lat1+lat2) / 2) * math.acos(a)
 
def MeterOffset((lat1, lon1), (lat2, lon2)):
    """
    Return offset in meters of second arg from first.
    """
    dx = EarthDistance((lat1, lon1), (lat1, lon2))
    dy = EarthDistance((lat1, lon1), (lat2, lon1))
    if lat1 < lat2:
        dy *= -1
    if lon1 < lon2:
        dx *= -1
    return (dx, dy)

installation

anonymous@undisclosed.example.com (Anonymous) — Tue, 20 Sep 2016 09:21:49 +0000

$ wget https://www.python.org/ftp/python/3.5.2/Python-3.5.2.tar.xz
$ tar -xf Python-3.5.2.tar.xz
$ rm Python-3.5.2.tar.xz
$ cd Python-3.5.2/
$ export LD_RUN_PATH=/usr/local/lib/
$ ./configure --enable-loadable-sqlite-extensions --enable-shared
$ make
# make install
$ cd ..
$ rm -Rf Python-3.5.2/

This is how I install Python on my Debian system. With shared libraries enabled and SQLite support.
SSL is supposed to be enabled by default with Python3.

lazy-vs-eager

anonymous@undisclosed.example.com (Anonymous) — Wed, 23 Jan 2013 14:01:55 +0000

Lazy vs Eager evaluation

Python generally uses an eager evaluation (in contrary to Haskell) but for example with Python 3 the range() function returns a special range object which computes elements of the list on demand.

>>> r = range(10)
>>> print(r)
range(0, 10)
>>> print(r[3])
3

This was not the case with Python 2 (without iterators):

>>> r = range(10)
>>> print r
[0, 1, 2, 3, 4, 5, 6, 7, 8, 9]
>>> print r[3]
3

Lazy evaluation saves execution time for large ranges.

native-datatypes

anonymous@undisclosed.example.com (Anonymous) — Tue, 11 Jan 2011 13:26:30 +0000

Booleans
Numbers
Strings
Bytes
Lists
Tuples
Sets
Dictionaries

Booleans

>>> x = 42
>>> 19 <= x <= 51
True
>>> 19 <= x <= 41
False
>>> 
>>> 0 == False
True
>>> a = 1
>>> a < 0
False
>>> a > 0
True
>>> 
>>> True + True
2
>>> True + False
1
>>> True * True
1
>>> True / False
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
ZeroDivisionError: int division or modulo by zero

>>> import this
>>> love = this
>>> this is love
True
>>> love is True
False
>>> love is False
False
>>> love is not True or False
True
>>> love is not True or False; love is love
True
True

Numbers

>>> type(42)
<class 'int'>
>>> isinstance(42, int)
True
>>> 
>>> type(1 + 1.0)
<class 'float'>
>>> 
>>> float(2)
2.0
>>> int(2.5) 
2
>>> int(-2.5)
-2
>>> 
>>> 1.12345678901234567890
1.1234567890123457
>>> type(1000000000000000000000)
<class 'int'>

>>> 11 /2
5.5
>>> 1 // 2   
0
>>> -11 // 2
-6
>>> 11.0 // 2
5.0
>>> 
>>> 11 ** 2
121
>>> 11 % 2
1

Lists

>>> a_list = ["a", 1, True, "Python", 3.6465]
>>> a_list[0]
'a'
>>> a_list[-1]
3.6465
>>> 
>>> a_list[:2]
['a', 1]
>>> a_list[2:]
[True, 'Python', 3.6465]
>>> a_list[1:4]
[1, True, 'Python']
>>> 
>>> 
>>> c = a_list[:2] + a_list[2:]
>>> c
['a', 1, True, 'Python', 3.6465]
>>> 
>>> d = a_list[:]
>>> d
['a', 1, True, 'Python', 3.6465]

>>> a_list = ["a", 1, True, "Python", 3.6465]
>>> a_list.append("Toto")
>>> a_list
['a', 1, True, 'Python', 3.6465, 'Toto']
>>> 
>>> a_list.extend([42, False])
>>> a_list
['a', 1, True, 'Python', 3.6465, 'Toto', 42, False]
>>> 
>>> a_list.insert(0, 'Ω')      
>>> a_list
['Ω', 'a', 1, True, 'Python', 3.6465, 'Toto', 42, False]
>>> 
>>> a_list.append((42, False))
>>> a_list
['Ω', 'a', 1, True, 'Python', 3.6465, 'Toto', 42, False, (42, False)]
>>> 
>>> len(a_list)
10
>>> a_list[-1]
(42, False)

Dictionaries

>>> a_dict = {'server': 'cedricbonhomme.org', 'database':'mysql'}
>>>
>>> a_dict['blog']
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
KeyError: 'blog'
>>> 'blog' in a_dict
False
>>> a_dict['blog'] = 'wordpress' 
>>> a_dict
{'blog': 'dotclear', 'database': 'mysql', 'server': 'cedricbonhomme.org'}
>>>
>>>
>>> a_dict.keys()
dict_keys(['blog', 'database', 'server'])
>>> a_dict.values()
dict_values(['dotclear', 'mysql', 'cedricbonhomme.org'])