Description of conditions, loops, etc. in Python

Gets any key-value pairs in the dictionary

>>> x={' A ': 1, ' B ':2}>>> key,value=x.popitem () >>> key,value (' A ', 1) >>> del X[key] Traceback (most recent):  File "<pyshell#16>", line 1, in <module>    del x[key]keyerror: ' A ' &G t;>> x{' B ': 2}>>> x[key]=value>>> x{' a ': 1, ' B ': 2}>>> del X[key]

Increment assignment

>>> x=2>>> x+=1>>> x*=2>>> x>>> fnord= ' foo ' >>> fnord+= ' bar ' >>> fnord*=2>>> fnord ' Foobarfoobar '

Conditional execution if statement

>>> name=raw_input ('? ')? Yq z>>> if Name.endswith (' Z '):  \   print ' Hello,mr.z ' Hello,mr.z

ELSE clause

>>> Name=raw_input (' What's your name? ') What is your name? Yq z>>> if Name.endswith (' Z '):    print ' hello,mr.z ' else:    print ' Hello,stranger '    hello,mr.z

elif clause

>>> num=input (' Enter a number: ') Enter a number:5>>> if num>0:    print ' The number is position ' Eli F num<0:    print ' The number is negative ' else:    print ' The number was zero ' the number is    position

Conditional nested statements

>>> Name=raw_input (' What's your name? ') What is your name? Yq z>>> if Name.endswith (' Yq '):    if Name.startswith (' Z '):        print ' Hello,yq z '    elif name.startswith (' K '): print '        hello,zyq '    else: print '        hello,yq ' else:    print ' Hello,stranger '    Hello, Stranger

>>> number=input (' Enter a number between 1 and: ') Enter a number between 1 and 10:6>>> if number<=1 0 and Number>=1:    print ' great! ' else:    print ' wrong! '    great!

>>> age=10>>> assert 0<age<100>>> age=-1>>> assert 0<age< 100Traceback (most recent):  File ' <pyshell#21> ', line 1, in <module>    assert 0<age< 100AssertionError

While loop

>>> x=1>>> while x<=100:    print x    x+=1

>>> While not name:    name=raw_input (' Please enter your name: ')    print ' hello,%s! '% name    Your NAME:ZYQHELLO,ZYQ!

For loop

>>> words=[' This ', ' was ', ' an ', ' ex ', ' Parrot ']>>> for word in words:    print word    Thisisanexparrot>>> Range (0,10) [0, 1, 2, 3, 4, 5, 6, 7, 8, 9]>>> for I in Range (1,8):    print i246

Dictionary loops (iterations)

>>> d={' x ': 1, ' Y ': 2, ' Z ':3}>>> for key in D:    print key, ' corresponds to ', D[key]    y corresponds to 2x corresponds to 1z corresponds to 3

Parallel iterations

>>> names=[' Anne ', ' Beth ', ' George ', ' Damon ']  >>> ages=[12,19,18,20]>>> for I in range ( Len (names)):    print names[i], "is", ages[i "," years old "Anne is" years Oldbeth is "years Oldgeorge is years    ol Ddamon is-years old

>>> Zip (names,ages) [(' Anne ', ' + '), (' Beth ', +), (' George ', '), (' Damon ', ')]>>> for name,age in Zip (NA mes,ages):    print name, ' is ', age, ' years-old ' Anne is-    years Oldbeth is years Oldgeorge-is-years Olddamon is Years old>>> Zip (range (5), xrange (100)) [(0, 0), (1, 1), (2, 2), (3, 3), (4, 4)]

Numbering iterations

>>> d[1, 2, 4, 4]>>> for x in D:    if x==4:        d[d.index (x)]=6        >>> d[1, 2, 6, 6]>> > s=[' skj ', ' Kiu ', ' Olm ', ' Piy ']>>> index=0>>> for S1 in S:    if ' K ' in S1:        s[index]= ' HH '    Index+=1    >>> s[' hh ', ' hh ', ' Olm ', ' Piy ']>>> for INDEX,S2 in Enumerate (S): #enumerate函数提供索引-value pairs    if ' H ' in S2:        s[index]= ' DF '        >>> s[' df ', ' df ', ' Olm ', ' Piy ']

Flipping, sorting iterations

>>> sorted ([4,3,6,8,3]) [3, 3, 4, 6, 8]>>> sorted (' hello,world! ') ['! ', ', ', ' H ', ' d ', ' e ', ' l ', ' l ', ' l ', ' o ', ' o ', ' R ', ' W ']>>> list (reversed (' hello,world! ')) ['! ', ' d ', ' l ', ' r ', ' O ', ' w ', ', ', ' o ', ' l ', ' l ', ' e ', ' H ']>>> '. Join (Reversed (' hello,world! ')) '! Dlrow,olleh '

Break jumps out of the loop

>>> for N in range (99,0,-1):    m=sqrt (n)    if M==int (m):        print n        break

While True/break

>>> while True:    word=raw_input (' Please enter a word: ')    if not word:break    print ' The word was ' +word< C7/>please Enter a word:fthe word was fplease enter a word:

Else statement in the loop

>>> for N in range (99,81,-1):    m=sqrt (n)    if M==int (m):        print M        breakelse:    print ' h '    h

List derivation-lightweight loops

>>> [x*x for x in range (][0), 1, 4, 9, +, +, $, $, a, 81]>>> [x*x for x in range] if x%3==0][0 , 9, 81]>>> [(x, Y) for x in range (3) for Y in range (3) [(0, 0), (0, 1), (0, 2), (, 1), (0, 1), (1, 1), (2), 0), (2, 1), (2, 2)]>>> result=[]>>> for x in range (3): For    y in range (3):        Result.append ((x, y)) &G t;>> result [(0, 0), (0, 1), (+ 0, 2), (1, 0), (1, 1), (1, 2), (2, 0), (2, 1), (2, 2)]>>> girls=[' Alice ', ' Ber Nice ', ' Clarice ']>>> boys=[' Chris ', ' Arnold ', ' Bob ']>>> [B + ' + ' +g for B in boys for g in girls if b[0]==g[0] [' Chris+clarice ', ' arnold+alice ', ' Bob+bernice ']

Pass

>>> if name== ' NSDs ':    print ' welcome! ' Elif name== ' UK ':    #还没完    passelif name== ' Bill ':    print ' Access Denied ' else:    print ' nobody! '

del X and Y point to a list at the same time, but deleting x does not affect Y. Delete only the name, not the list itself (value)

>>> x=[' hello ', ' World ']>>> y=x>>> y[1]= ' python ' >>> x[' hello ', ' python ']>> > del x>>> y[' Hello ', ' Python ']

Exec

>>> exec "print ' hello,world! '" hello,world!>>> from math import sqrt>>> exec "sqrt=1" >>> sqrt (4) Traceback (most recent call L AST):  File "<pyshell#36>", line 1, in <module>    sqrt (4) TypeError: ' int ' object was not callable# add a dictionary , play the role of the namespace >>> from math import sqrt>>> scope={}>>> exec ' sqrt=1 ' in scope>>> sqrt (4) 2.0>>> scope[' sqrt ']

Note: namespaces, called scopes. You can think of it as a place to save variables, similar to an invisible dictionary. When you execute an assignment such as x=1, the key x and value 1 are placed in the current namespace, which is generally the global namespace.

>>> len (Scope) 2>>> Scope.keys () [' __builtins__ ', ' sqrt ']

Eval evaluation

>>> scope={}>>> scope[' x ']=2>>> scope[' y ']=3>>> eval (' x*y ', scope) >>> scope={}>>> exec ' x=2 ' in scope>>> eval (' x*x ', scope)