Master Python Statements: From Basics to Advanced Control Flow

子曰：论笃是与？君子者乎？色庄者乎？——《论语》

Python Statement Formatting

Statement Start

In Python, statements are not delimited by braces; a new line ends a simple statement. Indentation must be consistent and cannot start with arbitrary spaces.

>> print(1)
  File "<stdin>", line 1
    print (1)
    ^
IndentationError: unexpected indent

Statement Alignment

In compound statements, indentation must be uniform.

>> def add(str):
...     str = str
...     print(str)
  File "<stdin>", line 3
    print(str)
        ^
IndentationError: unindent does not match any outer indentation

Compound Statements

Compound statements can be written in single-line or multi-line form. A colon starts the block.

# Single-line statement
>>> if 1>0: print(1)
... # Multi-line statement
>>> if 1>0:
...     int = 1
...     print(int)
1

Statement Termination

Simple statements end at the line break; multiple statements can be separated by semicolons.

>> a=3;b=3;print(a+b)
6

Error Handling

Use try to catch exceptions without stopping the program.

>> while True:
...     _input = input("please input digit:")
...     try:
...         print("{:d} *10 is {:d}".format(int(_input), int(_input)*10))
...         break
...     except:
...         print("{} is not a number")
please input digit:a
a is not a number
please input digit:1
1 *10 is 10

Variable Assignment

Naming Rules

Variable names may contain letters, digits, and underscores but must start with a letter or underscore. They are case‑sensitive and cannot be Python reserved words (e.g., False, class, def, if, raise, etc.). Special names surrounded by double underscores (e.g., __init__) have special meanings.

Printing

print Function

Since Python 3, print is a function returning None. Its signature is print(*objects, sep=' ', end='\n', file=sys.stdout).

>> print(1,2,3,sep=';')
1;2;3
>>> print(1,2,3,sep=':')
1:2:3
>>> print(1,2,3,end='')
1 2 3>>> print(4,5,6,end='')
456

Conditional Statements

Truth Testing

Expressions in if evaluate to True or False. Comparison, logical, membership, and identity operators are used.

>> 1<2
True
>>> True and True
True
>>> 1 in [1,2]
True
>>> 1 in {"1":1}
False

Logical Operators

and

returns the first falsy value or the last truthy value; or returns the first truthy value or the last falsy value.

>> False and [] and {}
[]
>>> 1 and 2 and True
True
>>> 0 or [1] or {"1":1}
[1]
>>> 0 or [] or {}
{}

Conditional Expression (Ternary)

>> 1 if True else 2
1
>>> 1 if False else 2
2
>>> 'True' if 1>2 else 'False'
'False'

Loops

while Loop

The loop continues while the test expression is true. Use break, continue, or modify the test variable to exit.

>> a=0;b=10
>>> while a<b:
...     print(a,end='')
...     a+=1
0123456789

break and continue

>> a=0;b=10
>>> while a<b:
...     a+=1
...     if a==3: continue
...     if a==7: break
...     print(a,end='')
12456

pass Statement

pass

acts as a placeholder where no action is needed.

>> if True:
...     print('true')
... else: pass
true

else Clause

The else block runs after a for or while loop completes normally.

>> a=0;b=10
>>> while a<b:
...     print(a,end='')
...     a+=1
... else:
...     print('end')
0123456789end

for Loop

for

iterates over any iterable object.

>> for i in a:
...     print(i,end='')
12345

Range Function

>> a=range(10)
>>> for i in a:
...     print(i,end='')
0123456789

Nested Loops

>> for l in [(1,2,3),(4,5,6),(7,8,9)]:
...     for i in l:
...         print(i,end='')
...     print()
123
456
789

Iterators and Comprehensions

File Iterator

File objects are their own iterators. Use .read(), .readline(), .readlines(), or the built‑in next() function.

>> file=open('1.txt')
>>> file.__next__()
'hello,world'
>>> next(file)
Traceback (most recent call last):
  StopIteration

Manual Iteration

>> li=[1,2,3]
>>> i=iter(li)
>>> next(i)
1
>>> next(i)
2
>>> next(i)
3
>>> next(i)
Traceback (most recent call last):
  StopIteration

Dictionary Iteration

>> dic={'a':1,'b':2,'c':3}
>>> for key in dic:
...     print(key,dic[key])
a 1
b 2
c 3

List Comprehension

>> li=[1,2,3,4,5]
>>> li=[i+10 for i in li]
[11, 12, 13, 14, 15]
>>> li=[i+10 for i in li if i!=13]
[11, 12, 14, 15]
>>> [x+y for x in [1,2,3] for y in [10,20,30]]
[11, 21, 31, 12, 22, 32, 13, 23, 33]

Other Iterable Utilities

Functions like map, sorted, enumerate, sum, any, all, max, and min operate on iterables.

>> list(map(str.upper, open('1.txt')))
['HELLO,WORLD']
>>> sorted(open('1.txt'))
['hello,world']
>>> list(enumerate(open('1.txt')))
[(0, 'hello,world')]
>>> max([3,5,1,6,4]), min([3,6,8,2,4])
(6, 2)
>>> any([1, [], 'True']), all([1, [], 'True'])
(True, False)

Generators in Python 3

Functions using yield produce lazy iterables.

>> def printlist(lst):
...     for i in lst:
...         print('function print:{}'.format(i))
...         yield 'Result: {}'.format(i)
>>> s=printlist([1,2,3])
>>> next(s)
function print:1
'Result: 1'
>>> next(s)
function print:2
'Result: 2'