Test if ProtoBuf value is using default

Question

I have a ProtoBuf object. I'd like to detect when a particular field is using the default, versus explicitly supplied. ``` message vector_measurement { measurement x = 1; measurement y = 2; measurement z = 3; } ... message measurement { ... float value = 2; ... } ``` When I use `HasField` it returns `True`, yet this is clearly not the case: ``` c = my_vector print(c) # x { # value: 60.3813476562 # } # y { # value: 0.444311201572 # } # z { # } print(c.x) # value: 60.3813476562 print(c.z) # print(c.z==None) # False print(c.z.value) # 0 print( c.HasField('x'), c.HasField('z') ) # (True, True ) print (c.z.HasField('value') ) # ValueError: Protocol message has no non-repeated submessage field "value" ``` The string representation seems to know that `z` is using a default value; how can I detect this myself?

Accepted Answer

You can check if a message is the default by calling `ByteSize()` on the message:

```
print(c.x.ByteSize())
# 5

print(c.z.ByteSize())
# 0
```

However, note that this is true for every default value, not just those not sent with the message. In other words, if each component value is explicitly set to exactly match the default, it will then report `ByteSize()==0`:

```
print(c.x, c.x.ByteSize())
#myBool: true
#stdDev: 1.06
#value: 14.32
# 12

c.x.myBool = False
print(c.x, c.x.ByteSize())
#stdDev: 1.06
#value: 14.32
# 10

c.x.value = 0
print(c.x, c.x.ByteSize())
#stdDev: 1.06
# 5

c.x.stdDev = 0
print(c.x, c.x.ByteSize())
# 0
```

There is no difference in ProtoBuf3 between an empty message and a message that has default values.

Test if ProtoBuf value is using default

Solution

Problem