What does mean in Python

Question

I see code like this for example in Python       if cnt  gt  0 and len aStr   gt  1          while cnt  gt  0                                aStr   aStr 1   aStr 0              cnt    1   What does the    mean

User · Answer

it means  append  THIS  to the current value   example   a    hello   a      world    printing a now will output   hello world

User · Answer

a    b is essentially the same as a   a   b  except that     always returns a newly allocated object  but    should  but doesn t have to  modify the object in-place if it s mutable  e g  list or dict  but int and str are immutable    In a   a   b  a is evaluated twice   Python  Simple Statements  A simple statement is comprised within a single logical line      If this is the first time you encounter the    operator  you may wonder why it matters that it may modify the object in-place instead of building a new one  Here is an example    two variables referring to the same list  gt  gt  gt  list1       gt  gt  gt  list2   list1       modifies the object pointed to by list1 and list2  gt  gt  gt  list1     0   gt  gt  gt  list1  list2   0    0        creates a new  independent object  gt  gt  gt  list1       gt  gt  gt  list2   list1  gt  gt  gt  list1   list1    0   gt  gt  gt  list1  list2   0

User · Answer

a    b   is in this case the same as  a   a   b   In this case cnt    1 means that cnt is increased by one   Note that the code you pasted will loop indefinitely if cnt   0 and len aStr    1   Edit  quote Carl Meyer         the answer is misleadingly mostly correct  There is a subtle but very significant difference between   and     see Bastien s answer

User · Answer

FYI  it looks like you might have an infinite loop in your example     if cnt  gt  0 and len aStr   gt  1      while cnt  gt  0                            aStr   aStr 1   aStr 0          cnt    1    a condition of entering the loop is that cnt is greater than 0 the loop continues to run as long as cnt is greater than 0 each iteration of the loop increments cnt by 1   The net result is that cnt will always be greater than 0 and the loop will never exit

User · Answer

is the in-place addition operator   It s the same as doing cnt   cnt   1  For example    gt  gt  gt  cnt   0  gt  gt  gt  cnt    2  gt  gt  gt  print cnt 2  gt  gt  gt  cnt    42  gt  gt  gt  print cnt 44   The operator is often used in a similar fashion to the    operator in C-ish languages  to increment a variable by one in a loop  i    1   There are similar operator for subtraction multiplication division power and others   i -  1   same as i   i - 1 i    2   i   i   2 i    3   i   i   3 i     4   i   i    4   The    operator also works on strings  for example    gt  gt  gt  s    Hi   gt  gt  gt  s      there   gt  gt  gt  print s Hi there   People tend to recommend against doing this for performance reason  but for the most scripts this really isn t an issue  To quote from the  Sequence Types  docs         If s and t are both strings  some Python implementations such as CPython can usually perform an in-place optimization for assignments of the form s s t or s  t  When applicable  this optimization makes quadratic run-time much less likely  This optimization is both version and implementation dependent  For performance sensitive code  it is preferable to use the str join   method which assures consistent linear concatenation performance across versions and implementations       The str join   method refers to doing the following   mysentence      for x in range 100       mysentence append  test       join mysentence      instead of the more obvious   mysentence      for x in range 100       mysentence      test    The problem with the later is  aside from the leading-space   depending on the Python implementation  the Python interpreter will have to make a new copy of the string in memory every time you append  because strings are immutable   which will get progressively slower the longer the string to append is   Whereas appending to a list then joining it together into a string is a consistent speed  regardless of implementation   If you re doing basic string manipulation  don t worry about it  If you see a loop which is basically just appending to a string  consider constructing an array  then    join   ing it

User · Answer

Google  python    operator  leads you to http   docs python org library operator html  Search for    once the page loads up for a more detailed answer

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