ch20迭代器和解析，第二部分（python）
>>> ord('s')
115
>>> res = []
>>> for x in 'spam':
... res.append(ord(x))
...
>>> res
[115, 112, 97, 109]
>>> res = list(map(ord, 'spam')) # Apply function to sequence
>>> res
[115, 112, 97, 109]
>>> res = [ord(x) for x in 'spam'] # Apply expression to sequence
>>> res
[115, 112, 97, 109]
>>> [x ** 2 for x in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> list(map((lambda x: x ** 2), range(10)))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> [x for x in range(5) if x % 2 == 0]
[0, 2, 4]
>>> list(filter((lambda x: x % 2 == 0), range(5)))
[0, 2, 4]
>>> res = []
>>> for x in range(5):
... if x % 2 == 0:
... res.append(x)
...
>>> res
[0, 2, 4]
>>> [x ** 2 for x in range(10) if x % 2 == 0]
[0, 4, 16, 36, 64]
>>> list( map((lambda x: x**2), filter((lambda x: x % 2 == 0), range(10))) )
[0, 4, 16, 36, 64]
>>> res = [x + y for x in [0, 1, 2] for y in [100, 200, 300]]
>>> res
[100, 200, 300, 101, 201, 301, 102, 202, 302]
>>> res = []
>>> for x in [0, 1, 2]:
... for y in [100, 200, 300]:
... res.append(x + y)
...
>>> res
[100, 200, 300, 101, 201, 301, 102, 202, 302]
>>> [x + y for x in 'spam' for y in 'SPAM']
['sS', 'sP', 'sA', 'sM', 'pS', 'pP', 'pA', 'pM',
'aS', 'aP', 'aA', 'aM', 'mS', 'mP', 'mA', 'mM']
>>> [(x, y) for x in range(5) if x % 2 == 0 for y in range(5) if y % 2 == 1]
[(0, 1), (0, 3), (2, 1), (2, 3), (4, 1), (4, 3)]
>>> res = []
>>> for x in range(5):
... if x % 2 == 0:
... for y in range(5):
... if y % 2 == 1:
... res.append((x, y))
...
>>> res
[(0, 1), (0, 3), (2, 1), (2, 3), (4, 1), (4, 3)]
>>> M = [[1, 2, 3],
... [4, 5, 6],
... [7, 8, 9]]
>>> N = [[2, 2, 2],
... [3, 3, 3],
... [4, 4, 4]]
>>> M[1]
[4, 5, 6]
>>> M[1][2]
6
>>> [row[1] for row in M]
[2, 5, 8]
>>> [M[row][1] for row in (0, 1, 2)]
[2, 5, 8]
>>> [M[i][i] for i in range(len(M))]
[1, 5, 9]
>>> [M[row][col] * N[row][col] for row in range(3) for col in range(3)]
[2, 4, 6, 12, 15, 18, 28, 32, 36]
>>> [[M[row][col] * N[row][col] for col in range(3)] for row in range(3)]
[[2, 4, 6], [12, 15, 18], [28, 32, 36]]
>>> res = []
>>> for row in range(3):
... tmp = []
... for col in range(3):
... tmp.append(M[row][col] * N[row][col])
... res.append(tmp)
...
>>> res
[[2, 4, 6], [12, 15, 18], [28, 32, 36]]
>>> open('myfile').readlines()
['aaa\n', 'bbb\n', 'ccc\n']
>>> [line.rstrip() for line in open('myfile').readlines()]
['aaa', 'bbb', 'ccc']
>>> [line.rstrip() for line in open('myfile')]
['aaa', 'bbb', 'ccc']
>>> list(map((lambda line: line.rstrip()), open('myfile')))
['aaa', 'bbb', 'ccc']
>>> listoftuple = [('bob', 35, 'mgr'), ('mel', 40, 'dev')]
>>> [age for (name, age, job) in listoftuple]
[35, 40]
>>> list(map((lambda row: row[1]), listoftuple))
[35, 40]
# 2.6 only
>>> list(map((lambda (name, age, job): age), listoftuple))
[35, 40]
>>> def gensquares(N):
... for i in range(N):
... yield i ** 2 # Resume here later
...
>>> for i in gensquares(5): # Resume the function
... print(i, end=' : ') # Print last yielded value
...
0 : 1 : 4 : 9 : 16 :
>>>
>>> x = gensquares(4)
>>> x
<generator object at 0x0086C378>
>>> next(x) # Same as x.__next__() in 3.0
0
>>> next(x) # Use x.next() or next() in 2.6
1
>>> next(x)
4
>>> next(x)
9
>>> next(x)
Traceback (most recent call last):
...more text omitted...
StopIteration
>>> def buildsquares(n):
... res = []
... for i in range(n): res.append(i ** 2)
... return res
...
>>> for x in buildsquares(5): print(x, end=' : ')
...
0 : 1 : 4 : 9 : 16 :
>>> for x in [n ** 2 for n in range(5)]:
... print(x, end=' : ')
...
0 : 1 : 4 : 9 : 16 :
>>> for x in map((lambda n: n ** 2), range(5)):
... print(x, end=' : ')
...
0 : 1 : 4 : 9 : 16 :
>>> def gen():
... for i in range(10):
... X = yield i
... print(X)
...
>>> G = gen()
>>> next(G) # Must call next() first, to start generator
0
>>> G.send(77) # Advance, and send value to yield expression
77
1
>>> G.send(88)
88
2
>>> next(G) # next() and X.__next__() send None
None
3
>>> [x ** 2 for x in range(4)] # List comprehension: build a list
[0, 1, 4, 9]
>>> (x ** 2 for x in range(4)) # Generator expression: make an iterable
<generator object at 0x011DC648>
>>> list(x ** 2 for x in range(4)) # List comprehension equivalence
[0, 1, 4, 9]
>>> G = (x ** 2 for x in range(4))
>>> next(G)
0
>>> next(G)
1
>>> next(G)
4
>>> next(G)
9
>>> next(G)
Traceback (most recent call last):
...more text omitted...
StopIteration
>>> for num in (x ** 2 for x in range(4)):
... print('%s, %s' % (num, num / 2.0))
...
0, 0.0
1, 0.5
4, 2.0
9, 4.5
>>> sum(x ** 2 for x in range(4))
14
>>> sorted(x ** 2 for x in range(4))
[0, 1, 4, 9]
>>> sorted((x ** 2 for x in range(4)), reverse=True)
[9, 4, 1, 0]
>>> import math
>>> list( map(math.sqrt, (x ** 2 for x in range(4))) )
[0.0, 1.0, 2.0, 3.0]
>>> G = (c * 4 for c in 'SPAM') # Generator expression
>>> list(G) # Force generator to produce all results
['SSSS', 'PPPP', 'AAAA', 'MMMM']
>>> def timesfour(S): # Generator function
... for c in S:
... yield c * 4
...
>>> G = timesfour('spam')
>>> list(G) # Iterate automatically
['ssss', 'pppp', 'aaaa', 'mmmm']
>>> G = (c * 4 for c in 'SPAM')
>>> I = iter(G) # Iterate manually
>>> next(I)
'SSSS'
>>> next(I)
'PPPP'
>>> G = timesfour('spam')
>>> I = iter(G)
>>> next(I)
'ssss'
>>> next(I)
'pppp'
>>> G = (c * 4 for c in 'SPAM')
>>> iter(G) is G # My iterator is myself: G has __next__
True
>>> G = (c * 4 for c in 'SPAM') # Make a new generator
>>> I1 = iter(G) # Iterate manually
>>> next(I1)
'SSSS'
>>> next(I1)
'PPPP'
>>> I2 = iter(G) # Second iterator at same position!
>>> next(I2)
'AAAA'
>>> list(I1) # Collect the rest of I1's items
['MMMM']
>>> next(I2) # Other iterators exhausted too
StopIteration
>>> I3 = iter(G) # Ditto for new iterators
>>> next(I3)
StopIteration
>>> I3 = iter(c * 4 for c in 'SPAM') # New generator to start over
>>> next(I3)
'SSSS'
>>> def timesfour(S):
... for c in S:
... yield c * 4
...
>>> G = timesfour('spam') # Generator functions work the same way
>>> iter(G) is G
True
>>> I1, I2 = iter(G), iter(G)
>>> next(I1)
'ssss'
>>> next(I1)
'pppp'
>>> next(I2) # I2 at same position as I1
'aaaa'
>>> L = [1, 2, 3, 4]
>>> I1, I2 = iter(L), iter(L)
>>> next(I1)
1
>>> next(I1)
2
>>> next(I2) # Lists support multiple iterators
1
>>> del L[2:] # Changes reflected in iterators
>>> next(I1)
StopIteration
>>> S1 = 'abc'
>>> S2 = 'xyz123'
>>> list(zip(S1, S2)) # zip pairs items from iterables
[('a', 'x'), ('b', 'y'), ('c', 'z')]
# zip pairs items, truncates at shortest
>>> list(zip([-2, -1, 0, 1, 2])) # Single sequence: 1-ary tuples
[(-2,), (-1,), (0,), (1,), (2,)]
>>> list(zip([1, 2, 3], [2, 3, 4, 5])) # N sequences: N-ary tuples
[(1, 2), (2, 3), (3, 4)]
# map passes paired itenms to a function, truncates
>>> list(map(abs, [-2, -1, 0, 1, 2])) # Single sequence: 1-ary function
[2, 1, 0, 1, 2]
>>> list(map(pow, [1, 2, 3], [2, 3, 4, 5])) # N sequences: N-ary function
[1, 8, 81]
# map(func, seqs...) workalike with zip
def mymap(func, *seqs):
res = []
for args in zip(*seqs):
res.append(func(*args))
return res
print(mymap(abs, [-2, -1, 0, 1, 2]))
print(mymap(pow, [1, 2, 3], [2, 3, 4, 5]))
# Using a list comprehension
def mymap(func, *seqs):
return [func(*args) for args in zip(*seqs)]
print(mymap(abs, [-2, -1, 0, 1, 2]))
print(mymap(pow, [1, 2, 3], [2, 3, 4, 5]))
# Using generators: yield and (...)
def mymap(func, *seqs):
res = []
for args in zip(*seqs):
yield func(*args)
def mymap(func, *seqs):
return (func(*args) for args in zip(*seqs))
print(list(mymap(abs, [-2, -1, 0, 1, 2])))
print(list(mymap(pow, [1, 2, 3], [2, 3, 4, 5])))
C:\misc> c:\python26\python
>>> map(None, [1, 2, 3], [2, 3, 4, 5])
[(1, 2), (2, 3), (3, 4), (None, 5)]
>>> map(None, 'abc', 'xyz123')
[('a', 'x'), ('b', 'y'), ('c', 'z'), (None, '1'), (None, '2'), (None, '3')]
# zip(seqs...) and 2.6 map(None, seqs...) workalikes
def myzip(*seqs):
seqs = [list(S) for S in seqs]
res = []
while all(seqs):
res.append(tuple(S.pop(0) for S in seqs))
return res
def mymapPad(*seqs, pad=None):
seqs = [list(S) for S in seqs]
res = []
while any(seqs):
res.append(tuple((S.pop(0) if S else pad) for S in seqs))
return res
S1, S2 = 'abc', 'xyz123'
print(myzip(S1, S2))
print(mymapPad(S1, S2))
print(mymapPad(S1, S2, pad=99))
# Using generators: yield
def myzip(*seqs):
seqs = [list(S) for S in seqs]
while all(seqs):
yield tuple(S.pop(0) for S in seqs)
def mymapPad(*seqs, pad=None):
seqs = [list(S) for S in seqs]
while any(seqs):
yield tuple((S.pop(0) if S else pad) for S in seqs)
S1, S2 = 'abc', 'xyz123'
print(list(myzip(S1, S2)))
print(list(mymapPad(S1, S2)))
print(list(mymapPad(S1, S2, pad=99)))
# Alternate implementation with lengths
def myzip(*seqs):
minlen = min(len(S) for S in seqs)
return [tuple(S[i] for S in seqs) for i in range(minlen)]
def mymapPad(*seqs, pad=None):
maxlen = max(len(S) for S in seqs)
index = range(maxlen)
return [tuple((S[i] if len(S) > i else pad) for S in seqs) for i in index]
S1, S2 = 'abc', 'xyz123'
print(myzip(S1, S2))
print(mymapPad(S1, S2))
print(mymapPad(S1, S2, pad=99))
# Using generators: (...)
def myzip(*seqs):
minlen = min(len(S) for S in seqs)
return (tuple(S[i] for S in seqs) for i in range(minlen))
print(list(myzip(S1, S2)))
def myzip(*args):
iters = map(iter, args)
while iters:
res = [next(i) for i in iters]
yield tuple(res)
>>> list(myzip('abc', 'lmnop'))
[('a', 'l'), ('b', 'm'), ('c', 'n')]
def myzip(*args):
iters = list(map(iter, args))
卹est as is?
>>> D = {'a':1, 'b':2, 'c':3}
>>> x = iter(D)
>>> next(x)
'a'
>>> next(x)
'c'
>>> for key in D:
... print(key, D[key])
...
a 1
c 3
b 2
>>> for line in open('temp.txt'):
... print(line, end='')
...
Tis but
a flesh wound.
>>> [x * x for x in range(10)] # List comprehension: builds list
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81] # like list(generator expr)
>>> (x * x for x in range(10)) # Generator expression: produces items
<generator object at 0x009E7328> # Parens are often optional
>>> {x * x for x in range(10)} # Set comprehension, new in 3.0
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36} # {x, y} is a set in 3.0 too
>>> {x: x * x for x in range(10)} # Dictionary comprehension, new in 3.0
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>> {x * x for x in range(10)} # Comprehension
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>> set(x * x for x in range(10)) # Generator and type name
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>> {x: x * x for x in range(10)}
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>> dict((x, x * x) for x in range(10))
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>> res = set()
>>> for x in range(10): # Set comprehension equivalent
... res.add(x * x)
...
>>> res
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>> res = {}
>>> for x in range(10): # Dict comprehension equivalent
... res[x] = x * x
...
>>> res
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>> G = ((x, x * x) for x in range(10))
>>> next(G)
(0, 0)
>>> next(G)
(1, 1)
>>> [x * x for x in range(10) if x % 2 == 0] # Lists are ordered
[0, 4, 16, 36, 64]
>>> {x * x for x in range(10) if x % 2 == 0} # But sets are not
{0, 16, 4, 64, 36}
>>> {x: x * x for x in range(10) if x % 2 == 0} # Neither are dict keys
{0: 0, 8: 64, 2: 4, 4: 16, 6: 36}
>>> [x + y for x in [1, 2, 3] for y in [4, 5, 6]] # Lists keep duplicates
[5, 6, 7, 6, 7, 8, 7, 8, 9]
>>> {x + y for x in [1, 2, 3] for y in [4, 5, 6]} # But sets do not
{8, 9, 5, 6, 7}
>>> {x: y for x in [1, 2, 3] for y in [4, 5, 6]} # Neither do dict keys
{1: 6, 2: 6, 3: 6}
>>> {x + y for x in 'ab' for y in 'cd'}
{'bd', 'ac', 'ad', 'bc'}
>>> {x + y: (ord(x), ord(y)) for x in 'ab' for y in 'cd'}
{'bd': (98, 100), 'ac': (97, 99), 'ad': (97, 100), 'bc': (98, 99)}
>>> {k * 2 for k in ['spam', 'ham', 'sausage'] if k[0] == 's'}
{'sausagesausage', 'spamspam'}
>>> {k.upper(): k * 2 for k in ['spam', 'ham', 'sausage'] if k[0] == 's'}
{'SAUSAGE': 'sausagesausage', 'SPAM': 'spamspam'}
### File: mytimer.py
import time
reps = 1000
repslist = range(reps)
def timer(func, *pargs, **kargs):
start = time.clock()
for i in repslist:
ret = func(*pargs, **kargs)
elapsed = time.clock() - start
return (elapsed, ret)
### File: timeseqs.py
import sys, mytimer # Import timer function
reps = 10000
repslist = range(reps) # Hoist range out in 2.6
def forLoop():
res = []
for x in repslist:
res.append(abs(x))
return res
def listComp():
return [abs(x) for x in repslist]
def mapCall():
return list(map(abs, repslist)) # Use list() in 3.0 only
def genExpr():
return list(abs(x) for x in repslist) # list() forces results
def genFunc():
def gen():
for x in repslist:
yield abs(x)
return list(gen())
print(sys.version)
for test in (forLoop, listComp, mapCall, genExpr, genFunc):
elapsed, result = mytimer.timer(test)
print ('-' * 33)
print ('%-9s: %.5f => [%s...%s]' %
(test.__name__, elapsed, result[0], result[-1]))
C:\misc> c:\python30\python timeseqs.py
### File: timeseqs.py (modification)
...
...
def forLoop():
res = []
for x in repslist:
res.append(x + 10)
return res
def listComp():
return [x + 10 for x in repslist]
def mapCall():
return list(map((lambda x: x + 10), repslist)) # list() in 3.0 only
def genExpr():
return list(x + 10 for x in repslist) # list() in 2.6 + 3.0
def genFunc():
def gen():
for x in repslist:
yield x + 10
return list(gen())
...
...
C:\misc> c:\python30\python timeseqs.py
### File: mytimer.py (2.6 and 3.0) (modified)
"""
timer(spam, 1, 2, a=3, b=4, _reps=1000) calls and times spam(1, 2, a=3)
_reps times, and returns total time for all runs, with final result;
best(spam, 1, 2, a=3, b=4, _reps=50) runs best-of-N timer to filter out
any system load variation, and returns best time among _reps tests
"""
import time, sys
if sys.platform[:3] == 'win':
timefunc = time.clock # Use time.clock on Windows
else:
timefunc = time.time # Better resolution on some Unix platforms
def trace(*args): pass # Or: print args
def timer(func, *pargs, **kargs):
_reps = kargs.pop('_reps', 1000) # Passed-in or default reps
trace(func, pargs, kargs, _reps)
repslist = range(_reps) # Hoist range out for 2.6 lists
start = timefunc()
for i in repslist:
ret = func(*pargs, **kargs)
elapsed = timefunc() - start
return (elapsed, ret)
def best(func, *pargs, **kargs):
_reps = kargs.pop('_reps', 50)
best = 2 ** 32
for i in range(_reps):
(time, ret) = timer(func, *pargs, _reps=1, **kargs)
if time < best: best = time
return (best, ret)
# File timeseqs.py (modifications)
import sys, mytimer
reps = 10000
repslist = range(reps)
def forLoop(): ...
def listComp(): ...
def mapCall(): ...
def genExpr(): ...
def genFunc(): ...
print(sys.version)
for tester in (mytimer.timer, mytimer.best):
print('<%s>' % tester.__name__)
for test in (forLoop, listComp, mapCall, genExpr, genFunc):
elapsed, result = tester(test)
print ('-' * 35)
print ('%-9s: %.5f => [%s...%s]' %
(test.__name__, elapsed, result[0], result[-1]))
C:\misc> c:\python30\python timeseqs.py
### File: mytimer.py (3.X only) (modified again)
"""
Use 3.0 keyword-only default arguments, instead of ** and dict pops.
No need to hoist range() out of test in 3.0: a generator, not a list
"""
import time, sys
trace = lambda *args: None # or print
timefunc = time.clock if sys.platform == 'win32' else time.time
def timer(func, *pargs, _reps=1000, **kargs):
trace(func, pargs, kargs, _reps)
start = timefunc()
for i in range(_reps):
ret = func(*pargs, **kargs)
elapsed = timefunc() - start
return (elapsed, ret)
def best(func, *pargs, _reps=50, **kargs):
best = 2 ** 32
for i in range(_reps):
(time, ret) = timer(func, *pargs, _reps=1, **kargs)
if time < best: best = time
return (best, ret)
C:\misc> c:\python30\python timeseqs.py
C:\misc> c:\python30\python
>>> from mytimer import timer, best
>>>
>>> def power(X, Y): return X ** Y # Test function
...
>>> timer(power, 2, 32) # Total time, last result
(0.002625403507987747, 4294967296)
>>> timer(power, 2, 32, _reps=1000000) # Override defult reps
(1.1822605247314932, 4294967296)
>>> timer(power, 2, 100000)[0] # 2 ** 100,000 tot time @1,000 reps
2.2496919999608878
>>> best(power, 2, 32) # Best time, last result
(5.58730229727189e-06, 4294967296)
>>> best(power, 2, 100000)[0] # 2 ** 100,000 best time
0.0019937589833460834
>>> best(power, 2, 100000, _reps=500)[0] # Override default reps
0.0019845399345541637
>>> timer(power, 2, 1000000, _reps=1)[0] # 2 ** 1,000,000: total time
0.088112804839710179
>>> timer(power, 2, 1000000, _reps=10)[0]
0.40922470593329763
>>> best(power, 2, 1000000, _reps=1)[0] # 2 ** 1,000,000: best time
0.086550036387279761
>>> best(power, 2, 1000000, _reps=10)[0] # 10 is sometimes as good as 50
0.029616752967200455
>>> best(power, 2, 1000000, _reps=50)[0] # Best resolution
0.029486918030102061
# NOTE: the indentation of the 2nd line was shifted in production
print('<%s>' % tester.__name__) # From expression
print('<{0}>'.format(tester.__name__)) # To method call
print ('%-9s: %.5f => [%s...%s]' %
(test.__name__, elapsed, result[0], result[-1]))
print('{0:<9}: {1:.5f} => [{2}...{3}]'.format(
test.__name__, elapsed, result[0], result[-1]))
>>> X = 99
>>> def selector(): # X used but not assigned
... print(X) # X found in global scope
...
>>> selector()
99
>>> def selector():
... print(X) # Does not yet exist!
... X = 88 # X classified as a local name (everywhere)
... # Can also happen for "import X", "def X"...
>>> selector()
...error text omitted...
UnboundLocalError: local variable 'X' referenced before assignment
>>> def selector():
... global X # Force X to be global (everywhere)
... print(X)
... X = 88
...
>>> selector()
99
>>> X = 99
>>> def selector():
... import __main__ # Import enclosing module
... print(__main__.X) # Qualify to get to global version of name
... X = 88 # Unqualified X classified as local
... print(X) # Prints local version of name
...
>>> selector()
99
88
>>> def saver(x=[]): # Saves away a list object
... x.append(1) # Changes same object each time!
... print(x)
...
>>> saver([2]) # Default not used
[2, 1]
>>> saver() # Default used
[1]
>>> saver() # Grows on each call!
[1, 1]
>>> saver()
[1, 1, 1]
>>> def saver(x=None):
... if x is None: # No argument passed?
... x = [] # Run code to make a new list
... x.append(1) # Changes new list object
... print(x)
...
>>> saver([2])
[2, 1]
>>> saver() # Doesn't grow here
[1]
>>> saver()
[1]
>>> def saver():
... saver.x.append(1)
... print(saver.x)
...
>>> saver.x = []
>>> saver()
[1]
>>> saver()
[1, 1]
>>> saver()
[1, 1, 1]
>>> def proc(x):
... print(x) # No return is a None return
...
>>> x = proc('testing 123...')
testing 123...
>>> print(x)
None
>>> list = [1, 2, 3]
>>> list = list.append(4) # append is a "procedure"
>>> print(list) # append changes list in-place
None
#### lab code
def f1(a, b): print(a, b) # Normal args
def f2(a, *b): print(a, b) # Positional varargs
def f3(a, **b): print(a, b) # Keyword varargs
def f4(a, *b, **c): print(a, b, c) # Mixed modes
def f5(a, b=2, c=3): print(a, b, c) # Defaults
def f6(a, b=2, *c): print(a, b, c) # Defaults and positional varargs
>>> f1(1, 2)
>>> f1(b=2, a=1)
>>> f2(1, 2, 3)
>>> f3(1, x=2, y=3)
>>> f4(1, 2, 3, x=2, y=3)
>>> f5(1)
>>> f5(1, 4)
>>> f6(1)
>>> f6(1, 3, 4)
x = y // 2 # For some y > 1
while x > 1:
if y % x == 0: # Remainder
print(y, 'has factor', x)
break # Skip else
x -= 1
else: # Normal exit
print(y, 'is prime')
115
>>> res = []
>>> for x in 'spam':
... res.append(ord(x))
...
>>> res
[115, 112, 97, 109]
>>> res = list(map(ord, 'spam')) # Apply function to sequence
>>> res
[115, 112, 97, 109]
>>> res = [ord(x) for x in 'spam'] # Apply expression to sequence
>>> res
[115, 112, 97, 109]
>>> [x ** 2 for x in range(10)]
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> list(map((lambda x: x ** 2), range(10)))
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81]
>>> [x for x in range(5) if x % 2 == 0]
[0, 2, 4]
>>> list(filter((lambda x: x % 2 == 0), range(5)))
[0, 2, 4]
>>> res = []
>>> for x in range(5):
... if x % 2 == 0:
... res.append(x)
...
>>> res
[0, 2, 4]
>>> [x ** 2 for x in range(10) if x % 2 == 0]
[0, 4, 16, 36, 64]
>>> list( map((lambda x: x**2), filter((lambda x: x % 2 == 0), range(10))) )
[0, 4, 16, 36, 64]
>>> res = [x + y for x in [0, 1, 2] for y in [100, 200, 300]]
>>> res
[100, 200, 300, 101, 201, 301, 102, 202, 302]
>>> res = []
>>> for x in [0, 1, 2]:
... for y in [100, 200, 300]:
... res.append(x + y)
...
>>> res
[100, 200, 300, 101, 201, 301, 102, 202, 302]
>>> [x + y for x in 'spam' for y in 'SPAM']
['sS', 'sP', 'sA', 'sM', 'pS', 'pP', 'pA', 'pM',
'aS', 'aP', 'aA', 'aM', 'mS', 'mP', 'mA', 'mM']
>>> [(x, y) for x in range(5) if x % 2 == 0 for y in range(5) if y % 2 == 1]
[(0, 1), (0, 3), (2, 1), (2, 3), (4, 1), (4, 3)]
>>> res = []
>>> for x in range(5):
... if x % 2 == 0:
... for y in range(5):
... if y % 2 == 1:
... res.append((x, y))
...
>>> res
[(0, 1), (0, 3), (2, 1), (2, 3), (4, 1), (4, 3)]
>>> M = [[1, 2, 3],
... [4, 5, 6],
... [7, 8, 9]]
>>> N = [[2, 2, 2],
... [3, 3, 3],
... [4, 4, 4]]
>>> M[1]
[4, 5, 6]
>>> M[1][2]
6
>>> [row[1] for row in M]
[2, 5, 8]
>>> [M[row][1] for row in (0, 1, 2)]
[2, 5, 8]
>>> [M[i][i] for i in range(len(M))]
[1, 5, 9]
>>> [M[row][col] * N[row][col] for row in range(3) for col in range(3)]
[2, 4, 6, 12, 15, 18, 28, 32, 36]
>>> [[M[row][col] * N[row][col] for col in range(3)] for row in range(3)]
[[2, 4, 6], [12, 15, 18], [28, 32, 36]]
>>> res = []
>>> for row in range(3):
... tmp = []
... for col in range(3):
... tmp.append(M[row][col] * N[row][col])
... res.append(tmp)
...
>>> res
[[2, 4, 6], [12, 15, 18], [28, 32, 36]]
>>> open('myfile').readlines()
['aaa\n', 'bbb\n', 'ccc\n']
>>> [line.rstrip() for line in open('myfile').readlines()]
['aaa', 'bbb', 'ccc']
>>> [line.rstrip() for line in open('myfile')]
['aaa', 'bbb', 'ccc']
>>> list(map((lambda line: line.rstrip()), open('myfile')))
['aaa', 'bbb', 'ccc']
>>> listoftuple = [('bob', 35, 'mgr'), ('mel', 40, 'dev')]
>>> [age for (name, age, job) in listoftuple]
[35, 40]
>>> list(map((lambda row: row[1]), listoftuple))
[35, 40]
# 2.6 only
>>> list(map((lambda (name, age, job): age), listoftuple))
[35, 40]
>>> def gensquares(N):
... for i in range(N):
... yield i ** 2 # Resume here later
...
>>> for i in gensquares(5): # Resume the function
... print(i, end=' : ') # Print last yielded value
...
0 : 1 : 4 : 9 : 16 :
>>>
>>> x = gensquares(4)
>>> x
<generator object at 0x0086C378>
>>> next(x) # Same as x.__next__() in 3.0
0
>>> next(x) # Use x.next() or next() in 2.6
1
>>> next(x)
4
>>> next(x)
9
>>> next(x)
Traceback (most recent call last):
...more text omitted...
StopIteration
>>> def buildsquares(n):
... res = []
... for i in range(n): res.append(i ** 2)
... return res
...
>>> for x in buildsquares(5): print(x, end=' : ')
...
0 : 1 : 4 : 9 : 16 :
>>> for x in [n ** 2 for n in range(5)]:
... print(x, end=' : ')
...
0 : 1 : 4 : 9 : 16 :
>>> for x in map((lambda n: n ** 2), range(5)):
... print(x, end=' : ')
...
0 : 1 : 4 : 9 : 16 :
>>> def gen():
... for i in range(10):
... X = yield i
... print(X)
...
>>> G = gen()
>>> next(G) # Must call next() first, to start generator
0
>>> G.send(77) # Advance, and send value to yield expression
77
1
>>> G.send(88)
88
2
>>> next(G) # next() and X.__next__() send None
None
3
>>> [x ** 2 for x in range(4)] # List comprehension: build a list
[0, 1, 4, 9]
>>> (x ** 2 for x in range(4)) # Generator expression: make an iterable
<generator object at 0x011DC648>
>>> list(x ** 2 for x in range(4)) # List comprehension equivalence
[0, 1, 4, 9]
>>> G = (x ** 2 for x in range(4))
>>> next(G)
0
>>> next(G)
1
>>> next(G)
4
>>> next(G)
9
>>> next(G)
Traceback (most recent call last):
...more text omitted...
StopIteration
>>> for num in (x ** 2 for x in range(4)):
... print('%s, %s' % (num, num / 2.0))
...
0, 0.0
1, 0.5
4, 2.0
9, 4.5
>>> sum(x ** 2 for x in range(4))
14
>>> sorted(x ** 2 for x in range(4))
[0, 1, 4, 9]
>>> sorted((x ** 2 for x in range(4)), reverse=True)
[9, 4, 1, 0]
>>> import math
>>> list( map(math.sqrt, (x ** 2 for x in range(4))) )
[0.0, 1.0, 2.0, 3.0]
>>> G = (c * 4 for c in 'SPAM') # Generator expression
>>> list(G) # Force generator to produce all results
['SSSS', 'PPPP', 'AAAA', 'MMMM']
>>> def timesfour(S): # Generator function
... for c in S:
... yield c * 4
...
>>> G = timesfour('spam')
>>> list(G) # Iterate automatically
['ssss', 'pppp', 'aaaa', 'mmmm']
>>> G = (c * 4 for c in 'SPAM')
>>> I = iter(G) # Iterate manually
>>> next(I)
'SSSS'
>>> next(I)
'PPPP'
>>> G = timesfour('spam')
>>> I = iter(G)
>>> next(I)
'ssss'
>>> next(I)
'pppp'
>>> G = (c * 4 for c in 'SPAM')
>>> iter(G) is G # My iterator is myself: G has __next__
True
>>> G = (c * 4 for c in 'SPAM') # Make a new generator
>>> I1 = iter(G) # Iterate manually
>>> next(I1)
'SSSS'
>>> next(I1)
'PPPP'
>>> I2 = iter(G) # Second iterator at same position!
>>> next(I2)
'AAAA'
>>> list(I1) # Collect the rest of I1's items
['MMMM']
>>> next(I2) # Other iterators exhausted too
StopIteration
>>> I3 = iter(G) # Ditto for new iterators
>>> next(I3)
StopIteration
>>> I3 = iter(c * 4 for c in 'SPAM') # New generator to start over
>>> next(I3)
'SSSS'
>>> def timesfour(S):
... for c in S:
... yield c * 4
...
>>> G = timesfour('spam') # Generator functions work the same way
>>> iter(G) is G
True
>>> I1, I2 = iter(G), iter(G)
>>> next(I1)
'ssss'
>>> next(I1)
'pppp'
>>> next(I2) # I2 at same position as I1
'aaaa'
>>> L = [1, 2, 3, 4]
>>> I1, I2 = iter(L), iter(L)
>>> next(I1)
1
>>> next(I1)
2
>>> next(I2) # Lists support multiple iterators
1
>>> del L[2:] # Changes reflected in iterators
>>> next(I1)
StopIteration
>>> S1 = 'abc'
>>> S2 = 'xyz123'
>>> list(zip(S1, S2)) # zip pairs items from iterables
[('a', 'x'), ('b', 'y'), ('c', 'z')]
# zip pairs items, truncates at shortest
>>> list(zip([-2, -1, 0, 1, 2])) # Single sequence: 1-ary tuples
[(-2,), (-1,), (0,), (1,), (2,)]
>>> list(zip([1, 2, 3], [2, 3, 4, 5])) # N sequences: N-ary tuples
[(1, 2), (2, 3), (3, 4)]
# map passes paired itenms to a function, truncates
>>> list(map(abs, [-2, -1, 0, 1, 2])) # Single sequence: 1-ary function
[2, 1, 0, 1, 2]
>>> list(map(pow, [1, 2, 3], [2, 3, 4, 5])) # N sequences: N-ary function
[1, 8, 81]
# map(func, seqs...) workalike with zip
def mymap(func, *seqs):
res = []
for args in zip(*seqs):
res.append(func(*args))
return res
print(mymap(abs, [-2, -1, 0, 1, 2]))
print(mymap(pow, [1, 2, 3], [2, 3, 4, 5]))
# Using a list comprehension
def mymap(func, *seqs):
return [func(*args) for args in zip(*seqs)]
print(mymap(abs, [-2, -1, 0, 1, 2]))
print(mymap(pow, [1, 2, 3], [2, 3, 4, 5]))
# Using generators: yield and (...)
def mymap(func, *seqs):
res = []
for args in zip(*seqs):
yield func(*args)
def mymap(func, *seqs):
return (func(*args) for args in zip(*seqs))
print(list(mymap(abs, [-2, -1, 0, 1, 2])))
print(list(mymap(pow, [1, 2, 3], [2, 3, 4, 5])))
C:\misc> c:\python26\python
>>> map(None, [1, 2, 3], [2, 3, 4, 5])
[(1, 2), (2, 3), (3, 4), (None, 5)]
>>> map(None, 'abc', 'xyz123')
[('a', 'x'), ('b', 'y'), ('c', 'z'), (None, '1'), (None, '2'), (None, '3')]
# zip(seqs...) and 2.6 map(None, seqs...) workalikes
def myzip(*seqs):
seqs = [list(S) for S in seqs]
res = []
while all(seqs):
res.append(tuple(S.pop(0) for S in seqs))
return res
def mymapPad(*seqs, pad=None):
seqs = [list(S) for S in seqs]
res = []
while any(seqs):
res.append(tuple((S.pop(0) if S else pad) for S in seqs))
return res
S1, S2 = 'abc', 'xyz123'
print(myzip(S1, S2))
print(mymapPad(S1, S2))
print(mymapPad(S1, S2, pad=99))
# Using generators: yield
def myzip(*seqs):
seqs = [list(S) for S in seqs]
while all(seqs):
yield tuple(S.pop(0) for S in seqs)
def mymapPad(*seqs, pad=None):
seqs = [list(S) for S in seqs]
while any(seqs):
yield tuple((S.pop(0) if S else pad) for S in seqs)
S1, S2 = 'abc', 'xyz123'
print(list(myzip(S1, S2)))
print(list(mymapPad(S1, S2)))
print(list(mymapPad(S1, S2, pad=99)))
# Alternate implementation with lengths
def myzip(*seqs):
minlen = min(len(S) for S in seqs)
return [tuple(S[i] for S in seqs) for i in range(minlen)]
def mymapPad(*seqs, pad=None):
maxlen = max(len(S) for S in seqs)
index = range(maxlen)
return [tuple((S[i] if len(S) > i else pad) for S in seqs) for i in index]
S1, S2 = 'abc', 'xyz123'
print(myzip(S1, S2))
print(mymapPad(S1, S2))
print(mymapPad(S1, S2, pad=99))
# Using generators: (...)
def myzip(*seqs):
minlen = min(len(S) for S in seqs)
return (tuple(S[i] for S in seqs) for i in range(minlen))
print(list(myzip(S1, S2)))
def myzip(*args):
iters = map(iter, args)
while iters:
res = [next(i) for i in iters]
yield tuple(res)
>>> list(myzip('abc', 'lmnop'))
[('a', 'l'), ('b', 'm'), ('c', 'n')]
def myzip(*args):
iters = list(map(iter, args))
卹est as is?
>>> D = {'a':1, 'b':2, 'c':3}
>>> x = iter(D)
>>> next(x)
'a'
>>> next(x)
'c'
>>> for key in D:
... print(key, D[key])
...
a 1
c 3
b 2
>>> for line in open('temp.txt'):
... print(line, end='')
...
Tis but
a flesh wound.
>>> [x * x for x in range(10)] # List comprehension: builds list
[0, 1, 4, 9, 16, 25, 36, 49, 64, 81] # like list(generator expr)
>>> (x * x for x in range(10)) # Generator expression: produces items
<generator object at 0x009E7328> # Parens are often optional
>>> {x * x for x in range(10)} # Set comprehension, new in 3.0
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36} # {x, y} is a set in 3.0 too
>>> {x: x * x for x in range(10)} # Dictionary comprehension, new in 3.0
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>> {x * x for x in range(10)} # Comprehension
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>> set(x * x for x in range(10)) # Generator and type name
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>> {x: x * x for x in range(10)}
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>> dict((x, x * x) for x in range(10))
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>> res = set()
>>> for x in range(10): # Set comprehension equivalent
... res.add(x * x)
...
>>> res
{0, 1, 4, 81, 64, 9, 16, 49, 25, 36}
>>> res = {}
>>> for x in range(10): # Dict comprehension equivalent
... res[x] = x * x
...
>>> res
{0: 0, 1: 1, 2: 4, 3: 9, 4: 16, 5: 25, 6: 36, 7: 49, 8: 64, 9: 81}
>>> G = ((x, x * x) for x in range(10))
>>> next(G)
(0, 0)
>>> next(G)
(1, 1)
>>> [x * x for x in range(10) if x % 2 == 0] # Lists are ordered
[0, 4, 16, 36, 64]
>>> {x * x for x in range(10) if x % 2 == 0} # But sets are not
{0, 16, 4, 64, 36}
>>> {x: x * x for x in range(10) if x % 2 == 0} # Neither are dict keys
{0: 0, 8: 64, 2: 4, 4: 16, 6: 36}
>>> [x + y for x in [1, 2, 3] for y in [4, 5, 6]] # Lists keep duplicates
[5, 6, 7, 6, 7, 8, 7, 8, 9]
>>> {x + y for x in [1, 2, 3] for y in [4, 5, 6]} # But sets do not
{8, 9, 5, 6, 7}
>>> {x: y for x in [1, 2, 3] for y in [4, 5, 6]} # Neither do dict keys
{1: 6, 2: 6, 3: 6}
>>> {x + y for x in 'ab' for y in 'cd'}
{'bd', 'ac', 'ad', 'bc'}
>>> {x + y: (ord(x), ord(y)) for x in 'ab' for y in 'cd'}
{'bd': (98, 100), 'ac': (97, 99), 'ad': (97, 100), 'bc': (98, 99)}
>>> {k * 2 for k in ['spam', 'ham', 'sausage'] if k[0] == 's'}
{'sausagesausage', 'spamspam'}
>>> {k.upper(): k * 2 for k in ['spam', 'ham', 'sausage'] if k[0] == 's'}
{'SAUSAGE': 'sausagesausage', 'SPAM': 'spamspam'}
### File: mytimer.py
import time
reps = 1000
repslist = range(reps)
def timer(func, *pargs, **kargs):
start = time.clock()
for i in repslist:
ret = func(*pargs, **kargs)
elapsed = time.clock() - start
return (elapsed, ret)
### File: timeseqs.py
import sys, mytimer # Import timer function
reps = 10000
repslist = range(reps) # Hoist range out in 2.6
def forLoop():
res = []
for x in repslist:
res.append(abs(x))
return res
def listComp():
return [abs(x) for x in repslist]
def mapCall():
return list(map(abs, repslist)) # Use list() in 3.0 only
def genExpr():
return list(abs(x) for x in repslist) # list() forces results
def genFunc():
def gen():
for x in repslist:
yield abs(x)
return list(gen())
print(sys.version)
for test in (forLoop, listComp, mapCall, genExpr, genFunc):
elapsed, result = mytimer.timer(test)
print ('-' * 33)
print ('%-9s: %.5f => [%s...%s]' %
(test.__name__, elapsed, result[0], result[-1]))
C:\misc> c:\python30\python timeseqs.py
### File: timeseqs.py (modification)
...
...
def forLoop():
res = []
for x in repslist:
res.append(x + 10)
return res
def listComp():
return [x + 10 for x in repslist]
def mapCall():
return list(map((lambda x: x + 10), repslist)) # list() in 3.0 only
def genExpr():
return list(x + 10 for x in repslist) # list() in 2.6 + 3.0
def genFunc():
def gen():
for x in repslist:
yield x + 10
return list(gen())
...
...
C:\misc> c:\python30\python timeseqs.py
### File: mytimer.py (2.6 and 3.0) (modified)
"""
timer(spam, 1, 2, a=3, b=4, _reps=1000) calls and times spam(1, 2, a=3)
_reps times, and returns total time for all runs, with final result;
best(spam, 1, 2, a=3, b=4, _reps=50) runs best-of-N timer to filter out
any system load variation, and returns best time among _reps tests
"""
import time, sys
if sys.platform[:3] == 'win':
timefunc = time.clock # Use time.clock on Windows
else:
timefunc = time.time # Better resolution on some Unix platforms
def trace(*args): pass # Or: print args
def timer(func, *pargs, **kargs):
_reps = kargs.pop('_reps', 1000) # Passed-in or default reps
trace(func, pargs, kargs, _reps)
repslist = range(_reps) # Hoist range out for 2.6 lists
start = timefunc()
for i in repslist:
ret = func(*pargs, **kargs)
elapsed = timefunc() - start
return (elapsed, ret)
def best(func, *pargs, **kargs):
_reps = kargs.pop('_reps', 50)
best = 2 ** 32
for i in range(_reps):
(time, ret) = timer(func, *pargs, _reps=1, **kargs)
if time < best: best = time
return (best, ret)
# File timeseqs.py (modifications)
import sys, mytimer
reps = 10000
repslist = range(reps)
def forLoop(): ...
def listComp(): ...
def mapCall(): ...
def genExpr(): ...
def genFunc(): ...
print(sys.version)
for tester in (mytimer.timer, mytimer.best):
print('<%s>' % tester.__name__)
for test in (forLoop, listComp, mapCall, genExpr, genFunc):
elapsed, result = tester(test)
print ('-' * 35)
print ('%-9s: %.5f => [%s...%s]' %
(test.__name__, elapsed, result[0], result[-1]))
C:\misc> c:\python30\python timeseqs.py
### File: mytimer.py (3.X only) (modified again)
"""
Use 3.0 keyword-only default arguments, instead of ** and dict pops.
No need to hoist range() out of test in 3.0: a generator, not a list
"""
import time, sys
trace = lambda *args: None # or print
timefunc = time.clock if sys.platform == 'win32' else time.time
def timer(func, *pargs, _reps=1000, **kargs):
trace(func, pargs, kargs, _reps)
start = timefunc()
for i in range(_reps):
ret = func(*pargs, **kargs)
elapsed = timefunc() - start
return (elapsed, ret)
def best(func, *pargs, _reps=50, **kargs):
best = 2 ** 32
for i in range(_reps):
(time, ret) = timer(func, *pargs, _reps=1, **kargs)
if time < best: best = time
return (best, ret)
C:\misc> c:\python30\python timeseqs.py
C:\misc> c:\python30\python
>>> from mytimer import timer, best
>>>
>>> def power(X, Y): return X ** Y # Test function
...
>>> timer(power, 2, 32) # Total time, last result
(0.002625403507987747, 4294967296)
>>> timer(power, 2, 32, _reps=1000000) # Override defult reps
(1.1822605247314932, 4294967296)
>>> timer(power, 2, 100000)[0] # 2 ** 100,000 tot time @1,000 reps
2.2496919999608878
>>> best(power, 2, 32) # Best time, last result
(5.58730229727189e-06, 4294967296)
>>> best(power, 2, 100000)[0] # 2 ** 100,000 best time
0.0019937589833460834
>>> best(power, 2, 100000, _reps=500)[0] # Override default reps
0.0019845399345541637
>>> timer(power, 2, 1000000, _reps=1)[0] # 2 ** 1,000,000: total time
0.088112804839710179
>>> timer(power, 2, 1000000, _reps=10)[0]
0.40922470593329763
>>> best(power, 2, 1000000, _reps=1)[0] # 2 ** 1,000,000: best time
0.086550036387279761
>>> best(power, 2, 1000000, _reps=10)[0] # 10 is sometimes as good as 50
0.029616752967200455
>>> best(power, 2, 1000000, _reps=50)[0] # Best resolution
0.029486918030102061
# NOTE: the indentation of the 2nd line was shifted in production
print('<%s>' % tester.__name__) # From expression
print('<{0}>'.format(tester.__name__)) # To method call
print ('%-9s: %.5f => [%s...%s]' %
(test.__name__, elapsed, result[0], result[-1]))
print('{0:<9}: {1:.5f} => [{2}...{3}]'.format(
test.__name__, elapsed, result[0], result[-1]))
>>> X = 99
>>> def selector(): # X used but not assigned
... print(X) # X found in global scope
...
>>> selector()
99
>>> def selector():
... print(X) # Does not yet exist!
... X = 88 # X classified as a local name (everywhere)
... # Can also happen for "import X", "def X"...
>>> selector()
...error text omitted...
UnboundLocalError: local variable 'X' referenced before assignment
>>> def selector():
... global X # Force X to be global (everywhere)
... print(X)
... X = 88
...
>>> selector()
99
>>> X = 99
>>> def selector():
... import __main__ # Import enclosing module
... print(__main__.X) # Qualify to get to global version of name
... X = 88 # Unqualified X classified as local
... print(X) # Prints local version of name
...
>>> selector()
99
88
>>> def saver(x=[]): # Saves away a list object
... x.append(1) # Changes same object each time!
... print(x)
...
>>> saver([2]) # Default not used
[2, 1]
>>> saver() # Default used
[1]
>>> saver() # Grows on each call!
[1, 1]
>>> saver()
[1, 1, 1]
>>> def saver(x=None):
... if x is None: # No argument passed?
... x = [] # Run code to make a new list
... x.append(1) # Changes new list object
... print(x)
...
>>> saver([2])
[2, 1]
>>> saver() # Doesn't grow here
[1]
>>> saver()
[1]
>>> def saver():
... saver.x.append(1)
... print(saver.x)
...
>>> saver.x = []
>>> saver()
[1]
>>> saver()
[1, 1]
>>> saver()
[1, 1, 1]
>>> def proc(x):
... print(x) # No return is a None return
...
>>> x = proc('testing 123...')
testing 123...
>>> print(x)
None
>>> list = [1, 2, 3]
>>> list = list.append(4) # append is a "procedure"
>>> print(list) # append changes list in-place
None
#### lab code
def f1(a, b): print(a, b) # Normal args
def f2(a, *b): print(a, b) # Positional varargs
def f3(a, **b): print(a, b) # Keyword varargs
def f4(a, *b, **c): print(a, b, c) # Mixed modes
def f5(a, b=2, c=3): print(a, b, c) # Defaults
def f6(a, b=2, *c): print(a, b, c) # Defaults and positional varargs
>>> f1(1, 2)
>>> f1(b=2, a=1)
>>> f2(1, 2, 3)
>>> f3(1, x=2, y=3)
>>> f4(1, 2, 3, x=2, y=3)
>>> f5(1)
>>> f5(1, 4)
>>> f6(1)
>>> f6(1, 3, 4)
x = y // 2 # For some y > 1
while x > 1:
if y % x == 0: # Remainder
print(y, 'has factor', x)
break # Skip else
x -= 1
else: # Normal exit
print(y, 'is prime')
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