Python s equivalent of logical-and in an if-statement

Question

Here s my code   def front back a  b          your code here      if len a    2    0  amp  amp  len b    2    0      return a   len a  2     b   len b  2     a  len a  2      b  len b  2       else       todo  Not yet done   P   return   I m getting an error in the IF conditional  What am I doing wrong

User · Answer

Two comments    Use and and or for logical operations in Python  Use 4 spaces to indent instead of 2   You will thank yourself later because your code will look pretty much the same as everyone else s code   See PEP 8 for more details

User · Answer

A single  amp   not double  amp  amp   is enough or as the top answer suggests you can use  and   I also found this in pandas   cities  Is wide and has saint name      cities  Population    gt  1000000    amp  cities  City name   apply lambda name  name startswith  San      if we replace the   amp   with  and   it won t work

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maybe with  amp  instead   is more fast and mantain readibility  other tests even odd  x is even     x   2    0  x is odd      not x   2    0  maybe is more clear with bitwise and 1  x is odd      x  amp  1  x is even     not x  amp  1     not odd   def front back a  b            your code here        if not len a   amp  1 and not len b   amp  1          return a   len a  2     b   len b  2     a  len a  2      b  len b  2         else           todo  Not yet done   P     return

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Probably this is not best code for this task  but is working -  def front back a  b     if len a    2    0 and len b    2    0      print a   len a  2     b   len b  2     a  len a  2      b  len b  2      elif len a    2    1 and len b    2    0      print a   len a  2  1    b   len b  2     a  len a  2  1     b  len b  2       elif len a    2    0 and len b    2    1       print a   len a  2     b   len b  2  1    a  len a  2      b  len b  2  1      else        print a   len a  2  1    b   len b  2  1    a  len a  2  1     b  len b  2  1

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Python uses and and or conditionals   i e   if foo     abc  and bar     bac  or zoo     123       do something

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Use of  and  in conditional  I often use this when importing in Jupyter Notebook   def find local py scripts        import os   does not cost if already imported     for entry in os scandir                 find files ending with  py         if entry is file   and entry name endswith   py                 print  -    entry name  find local py scripts    -  googlenet custom layers py -  GoogLeNet Inception v1 py

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I m getting an error in the IF conditional  What am I doing wrong    There reason that you get a SyntaxError is that there is no  amp  amp  operator in Python  Likewise    and   are not valid Python operators    Some of the operators you may know from other languages have a different name in Python  The logical operators  amp  amp  and    are actually called and and or  Likewise the logical negation operator   is called not   So you could just write   if len a    2    0 and len b    2    0    or even   if not  len a    2 or len b    2     Some additional information  that might come in handy    I summarized the operator  equivalents  in this table    ------------------------------ ---------------------     Operator  other languages      Operator  Python                                                                            amp  amp                         and            ------------------------------ ---------------------                                           or             ------------------------------ ---------------------                                           not            ------------------------------ ---------------------    See also Python documentation  6 11  Boolean operations   Besides the logical operators Python also has bitwise binary operators    -------------------- --------------------     Logical operator     Bitwise operator                                                         and                    amp              -------------------- --------------------            or                                 -------------------- --------------------    There is no bitwise negation in Python  just the bitwise inverse operator   - but that is not equivalent to not    See also 6 6  Unary arithmetic and bitwise binary operations and 6 7  Binary arithmetic operations   The logical operators  like in many other languages  have the advantage that these are short-circuited  That means if the first operand already defines the result  then the second operator isn t evaluated at all   To show this I use a function that simply takes a value  prints it and returns it again  This is handy to see what is actually evaluated because of the print statements    gt  gt  gt  def print and return value           print value          return value   gt  gt  gt  res   print and return False  and print and return True  False   As you can see only one print statement is executed  so Python really didn t even look at the right operand   This is not the case for the binary operators  Those always evaluate both operands    gt  gt  gt  res   print and return False   amp  print and return True   False True   But if the first operand isn t enough then  of course  the second operator is evaluated    gt  gt  gt  res   print and return True  and print and return False   True False   To summarize this here is another Table    ----------------- -------------------------      Expression       Right side evaluated                                                      True  and                  Yes              ----------------- -------------------------     False  and                 No               ----------------- -------------------------      True  or                  No               ----------------- -------------------------     False  or                  Yes              ----------------- -------------------------    The True and False represent what bool left-hand-side  returns  they don t have to be True or False  they just need to return True or False when bool is called on them  1    So in Pseudo-Code    the and and or functions work like these   def and expr1  expr2       left   evaluate expr1      if bool left           return evaluate expr2      else          return left  def or expr1  expr2       left   evaluate expr1      if bool left           return left     else          return evaluate expr2    Note that this is pseudo-code not Python code  In Python you cannot create functions called and or or because these are keywords   Also you should never use  evaluate  or if bool        Customizing the behavior of your own classes  This implicit bool call can be used to customize how your classes behave with and  or and not   To show how this can be customized I use this class which again prints something to track what is happening   class Test object       def   init   self  value           self value   value      def   bool   self           print    bool   called on   r   format self           return bool self value         nonzero       bool      Python 2 compatibility      def   repr   self           return   self   class     name     self value    format self self    So let s see what happens with that class in combination with these operators    gt  gt  gt  if Test True  and Test False           pass   bool   called on Test True    bool   called on Test False    gt  gt  gt  if Test False  or Test False           pass   bool   called on Test False    bool   called on Test False    gt  gt  gt  if not Test True           pass   bool   called on Test True    If you don t have a   bool   method then Python also checks if the object has a   len   method and if it returns a value greater than zero   That might be useful to know in case you create a sequence container   See also 4 1  Truth Value Testing   NumPy arrays and subclasses  Probably a bit beyond the scope of the original question but in case you re dealing with NumPy arrays or subclasses  like Pandas Series or DataFrames  then the implicit bool call will raise the dreaded ValueError    gt  gt  gt  import numpy as np  gt  gt  gt  arr   np array  1 2 3    gt  gt  gt  bool arr  ValueError  The truth value of an array with more than one element is ambiguous  Use a any   or a all    gt  gt  gt  arr and arr ValueError  The truth value of an array with more than one element is ambiguous  Use a any   or a all     gt  gt  gt  import pandas as pd  gt  gt  gt  s   pd Series  1 2 3    gt  gt  gt  bool s  ValueError  The truth value of a Series is ambiguous  Use a empty  a bool    a item    a any   or a all     gt  gt  gt  s and s ValueError  The truth value of a Series is ambiguous  Use a empty  a bool    a item    a any   or a all      In these cases you can use the logical and function from NumPy which performs an element-wise and  or or     gt  gt  gt  np logical and np array  False False True True    np array  True  False  True  False    array  False  False   True  False    gt  gt  gt  np logical or np array  False False True True    np array  True  False  True  False    array   True  False   True   True     If you re dealing just with boolean arrays you could also use the binary operators with NumPy  these do perform element-wise  but also binary  comparisons    gt  gt  gt  np array  False False True True    amp  np array  True  False  True  False   array  False  False   True  False    gt  gt  gt  np array  False False True True     np array  True  False  True  False   array   True  False   True   True        1   That the bool call on the operands has to return True or False isn t completely correct  It s just the first operand that needs to return a boolean in it s   bool   method   class Test object       def   init   self  value           self value   value      def   bool   self           return self value        nonzero       bool      Python 2 compatibility      def   repr   self           return   self   class     name     self value    format self self    gt  gt  gt  x   Test 10  and Test 10  TypeError    bool   should return bool  returned int  gt  gt  gt  x1   Test True  and Test 10   gt  gt  gt  x2   Test False  and Test 10    That s because and actually returns the first operand if the first operand evaluates to False and if it evaluates to True then it returns the second operand    gt  gt  gt  x1 Test 10   gt  gt  gt  x2 Test False    Similarly for or but just the other way around    gt  gt  gt  Test True  or Test 10  Test True   gt  gt  gt  Test False  or Test 10  Test 10    However if you use them in an if statement the if will also implicitly call bool on the result  So these finer points may not be relevant for you

User · Answer

You would want and instead of  amp  amp

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I went with a purlely mathematical solution   def front back a  b     return a   len a  1   2  b   len b  1   2  a  len a  1   2   b  len b  1   2

User · Answer

You use and and or to perform logical operations like in C  C    Like literally and is  amp  amp  and or is         Take a look at this fun example   Say you want to build Logic Gates in Python    def AND a b       return  a and b   using and operator  def OR a b       return  a or b    using or operator   Now try calling them    print AND False  False  print OR True  False    This will output   False True   Hope this helps

[python] Python's equivalent of && (logical-and) in an if-statement

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