Round float to x decimals

Question

Is there a way to round a python float to x decimals  For example    gt  gt  gt  x   roundfloat 66 66666666666  4  66 6667  gt  gt  gt x   roundfloat 1 29578293  6  1 295783   I ve found ways to trim truncate them  66 666666666 --  66 6666   but not round  66 666666666 --  66 6667

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I coded a function  used in Django project for DecimalField  but it can be used in Python project   This code    Manage integers digits to avoid too high number Manage decimals digits to avoid too low number Manage signed and unsigned numbers  Code with tests   def convert decimal to right value  max digits  decimal places  signed True        integer digits   max digits - decimal places     max value   float  10  integer digits -float float 1  float  10  decimal places          if signed          min value   max value -1     else          min value   0      if value  gt  max value          value   max value      if value  lt  min value          value   min value      return round value  decimal places    value   12 12345 nb   convert decimal to right value  4  2    nb   12 12  value   12 126 nb   convert decimal to right value  4  2    nb   12 13  value   1234 123 nb   convert decimal to right value  4  2    nb   99 99  value   -1234 123 nb   convert decimal to right value  4  2    nb   -99 99  value   -1234 123 nb   convert decimal to right value  4  2  signed   False    nb   0  value   12 123 nb   convert decimal to right value  8  4    nb   12 123

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Default rounding in python and numpy  In   round i  for i in np arange 10     5  Out   0  2  2  4  4  6  6  8  8  10   I used this to get integer rounding to be applied to a pandas series  import decimal and use this line to set the rounding to  quot half up quot  a k a rounding as taught in school  decimal getcontext   rounding   decimal ROUND HALF UP Finally I made this function to apply it to a pandas series object def roundint value       return value apply lambda x  int decimal Decimal x  to integral value      So now you can do roundint df columnname  And for numbers  In   int decimal Decimal i  to integral value    for i in np arange 10     5  Out   1  2  3  4  5  6  7  8  9  10   Credit  kares

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Use the built-in function round     In  23   round 66 66666666666 4  Out 23   66 6667  In  24   round 1 29578293 6  Out 24   1 295783   help on round        round number   ndigits   -  floating point number      Round a number to a given precision in decimal digits  default 0   digits   This always returns a floating point number   Precision may   be negative

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The Mark Dickinson answer  although complete  didn t work with the float 52 15  case  After some tests  there is the solution that I m using  import decimal          def value to decimal value  decimal places       decimal getcontext   rounding   decimal ROUND HALF UP    define rounding method     return decimal Decimal str float value    quantize decimal Decimal  1e-    format decimal places      The conversion of the  value  to float and then string is very important  that way   value  can be of the type float  decimal  integer or string   Hope this helps anyone

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I feel compelled to provide a counterpoint to Ashwini Chaudhary s answer   Despite appearances  the two-argument form of the round function does not round a Python float to a given number of decimal places  and it s often not the solution you want  even when you think it is  Let me explain     The ability to round a  Python  float to some number of decimal places is something that s frequently requested  but turns out to be rarely what s actually needed   The beguilingly simple answer round x  number of places  is something of an attractive nuisance  it looks as though it does what you want  but thanks to the fact that Python floats are stored internally in binary  it s doing something rather subtler   Consider the following example    gt  gt  gt  round 52 15  1  52 1   With a naive understanding of what round does  this looks wrong   surely it should be rounding up to 52 2 rather than down to 52 1   To understand why such behaviours can t be relied upon  you need to appreciate that while this looks like a simple decimal-to-decimal operation  it s far from simple   So here s what s really happening in the example above   deep breath  We re displaying a decimal representation of the nearest binary floating-point number to the nearest n-digits-after-the-point decimal number to a binary floating-point approximation of a numeric literal written in decimal   So to get from the original numeric literal to the displayed output  the underlying machinery has made four separate conversions between binary and decimal formats  two in each direction   Breaking it down  and with the usual disclaimers about assuming IEEE 754 binary64 format  round-ties-to-even rounding  and IEEE 754 rules     First the numeric literal 52 15 gets parsed and converted to a Python float   The actual number stored is 7339460017730355   2  -47  or 52 14999999999999857891452847979962825775146484375  Internally as the first step of the round operation  Python computes the closest 1-digit-after-the-point decimal string to the stored number   Since that stored number is a touch under the original value of 52 15  we end up rounding down and getting a string 52 1   This explains why we re getting 52 1 as the final output instead of 52 2  Then in the second step of the round operation  Python turns that string back into a float  getting the closest binary floating-point number to 52 1  which is now 7332423143312589   2  -47  or 52 10000000000000142108547152020037174224853515625  Finally  as part of Python s read-eval-print loop  REPL   the floating-point value is displayed  in decimal    That involves converting the binary value back to a decimal string  getting 52 1 as the final output    In Python 2 7 and later  we have the pleasant situation that the two conversions in step 3 and 4 cancel each other out   That s due to Python s choice of repr implementation  which produces the shortest decimal value guaranteed to round correctly to the actual float   One consequence of that choice is that if you start with any  not too large  not too small  decimal literal with 15 or fewer significant digits then the corresponding float will be displayed showing those exact same digits    gt  gt  gt  x   15 34509809234  gt  gt  gt  x 15 34509809234   Unfortunately  this furthers the illusion that Python is storing values in decimal   Not so in Python 2 6  though  Here s the original example executed in Python 2 6    gt  gt  gt  round 52 15  1  52 200000000000003   Not only do we round in the opposite direction  getting 52 2 instead of 52 1  but the displayed value doesn t even print as 52 2   This behaviour has caused numerous reports to the Python bug tracker along the lines of  round is broken     But it s not round that s broken  it s user expectations    Okay  okay  round is a little bit broken in Python 2 6  in that it doesn t use correct rounding    Short version  if you re using two-argument round  and you re expecting predictable behaviour from a binary approximation to a decimal round of a binary approximation to a decimal halfway case  you re asking for trouble   So enough with the  two-argument round is bad  argument   What should you be using instead   There are a few possibilities  depending on what you re trying to do    If you re rounding for display purposes  then you don t want a float result at all   you want a string   In that case the answer is to use string formatting    gt  gt  gt  format 66 66666666666    4f    66 6667   gt  gt  gt  format 1 29578293    6f    1 295783    Even then  one has to be aware of the internal binary representation in order not to be surprised by the behaviour of apparent decimal halfway cases    gt  gt  gt  format 52 15    1f    52 1   If you re operating in a context where it matters which direction decimal halfway cases are rounded  for example  in some financial contexts   you might want to represent your numbers using the Decimal type   Doing a decimal round on the Decimal type makes a lot more sense than on a binary type  equally  rounding to a fixed number of binary places makes perfect sense on a binary type    Moreover  the decimal module gives you better control of the rounding mode   In Python 3  round does the job directly   In Python 2  you need the quantize method    gt  gt  gt  Decimal  66 66666666666   quantize Decimal  1e-4    Decimal  66 6667    gt  gt  gt  Decimal  1 29578293   quantize Decimal  1e-6    Decimal  1 295783    In rare cases  the two-argument version of round really is what you want  perhaps you re binning floats into bins of size 0 01  and you don t particularly care which way border cases go   However  these cases are rare  and it s difficult to justify the existence of the two-argument version of the round builtin based on those cases alone

[python] Round float to x decimals?

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