python ValueError invalid literal for float

Question

I ve a script which reads temperature data   def get temp socket  channels        data          for ch in channels          socket sendall  KRDG   s n    ch          time sleep 0 2          temp   socket recv 32  rstrip   r n            data ch    float temp    Sometimes  the script fails on the line which converts the values to float      File    projector py   line 129  in get temp   data ch    float temp    ValueError  invalid literal for float     135 057E 0    078 260E 0    00029   but this is NOT an invalid literal  If I enter this into any python shell   float  135 057E 0    then it correctly returns 135 057   So what is the problem

User · Answer

I had a similar issue reading the serial output from a digital scale. I was reading [3:12] out of a 18 characters long output string.

In my case sometimes there is a null character "\x00" (NUL) which magically appears in the scale's reply string and is not printed.

I was getting the error:

> '     0.00'
> 3 0 fast loop, delta =  10.0 weight =  0.0 
> '     0.00'
> 1 800 fast loop, delta = 10.0 weight =  0.0 
> '     0.00'
> 6 0 fast loop, delta =  10.0 weight =  0.0
> '     0\x00.0' 
> Traceback (most recent call last):
>   File "measure_weight_speed.py", line 172, in start
>     valueScale = float(answer_string) 
>     ValueError: invalid literal for float(): 0

After some research I wrote few lines of code that work in my case.

replyScale = scale_port.read(18)
answer = replyScale[3:12]
answer_decode = answer.replace("\x00", "")
answer_strip = str(answer_decode.strip())
print(repr(answer_strip))
valueScale = float(answer_strip)

The answers in these posts helped:

User · Answer

I would all but guarantee that the issue is some sort of non-printing character that s present in the value you pulled off your socket   It looks like you re using Python 2 x  in which case you can check for them with this   print repr temp    You ll likely see something in there that s escaped in the form  x00   These non-printing characters don t show up when you print directly to the console  but their presence is enough to negatively impact the parsing of a string value into a float   -- Edited for question changes --   It turns this is partly accurate for your issue - the root cause however appears to be that you re reading more information than you expect from your socket or otherwise receiving multiple values   You could do something like   map float  temp strip   split   r n      In order to convert each of the values  but if your function is supposed to return a single float value this is likely to cause confusion   Anyway  the issue certainly revolves around the presence of characters you did not expect to see in the value you retrieved from your socket

User · Answer

Watch out for possible unintended literals in your argument   for example you can have a space within your argument  rendering it to a string   literal   float   0 33     After making sure the unintended space did not make it into the argument  I was left with   float 0 33     Like this it works like a charm    Take away is  Pay Attention for unintended literals  e g  spaces that you didn t see  within your input

[python] python ValueError: invalid literal for float()

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